Here is a compendium of trouble shooting techniques for Macs. See the Repair Stories page for some descriptions of problem solving.

Things to Try

Troubleshoot a Crashing Mac Program

If you have a program that crashes when you run it:

1. Try it again. Write down exactly what you did and what you observed.
2. Check that you have the latest version. When did you last use it successfully? What changed since then?
3. Restart your computer, and try the program again.
4. Make sure you have at least 1GB free disk space. OS X does funny things when it is nearly out of disk space.
5. Google to see if others have this problem and how they fixed it. If the software vendor has a user forum, look there for similar problems. (Understand that there is a lot of wrong information out there.)
6. Find the preferences for the program in ~/Library/Preferences, move them to the Desktop, and try again. Sometimes preferences files get corrupted.
7. Log out and log into another account (create one if necessary), and try the program again. If this works, then there is some feature of your account that is causing the problem.
8. Restart your computer holding down the shift key to start in safe mode, and try the program again. If this works, then some other software you installed on your machine is conflicting with your program.
9. Use the Console utility to check for log messages and crash dumps related to your program. This may suggest what is failing.
10. Reinstall the program.
11. Contact the software vendor's support.
12. Boot from the OS X install disc and repair your hard disk with Disk Utility, and try the program again.
13. Boot from the OS X install disc while holding down the shift key to start in safe mode, and try the program again.
14. Boot from the OS X install disc while holding down the D key to run hardware diagnostics.
15. Boot from the OS X install disc while holding down the C key to reinstall the OS, and try the program again.

Can't connect to the Internet

If you are having an Internet problem, for example if there is "connection failure" or a little exclamation in a triangle next to Inbox when reading mail, or the web browser displays a "can't connect" message:

1. Can another local computer read mail or access the web? If so, check the Ethernet cable on the failing computer, and consider restarting it.
2. Exit and restart your mail program or web browser in case it is hung up. Try it again.
3. Try loading an external page like google.com in a web browser. If that works but mail does not, then the problem is likely at the mail host. Their hardware may be down, or the POP or SMTP server processes on the mail host may be down. Usually such problems are fixed in less than an hour. Check their server status page or call support if it's urgent.
4. If external pages won't load, exit and restart the browser, check connections, and restart the computer. Try an external web page again after each step.
5. If google.com won't load, try typing 74.125.45.100 in the address bar. If this works, then there is a name server problem at the Cable/DSL provider. Happens to me about once a year. Often they fix it within an hour. If you can't wait, then change the name server in System Preferences to OpenDNS (208.67.222.222, 208.67.220.220).
6. Check  ► System Preferences ► Network to make sure you have an IP address like 192.168.1.103. If not, perhaps your firewall router is not providing a DHCP address, or the Apple firewall preferences have been corrupted. (If you are connecting by AirPort, see "Wireless Network Problem" below.) Check that all Ethernet cables are plugged in. Turn off the power to the router and modem. Wait 60 seconds and turn it back on. The modem and router should go through about a minute of flashing lights before they settle down. Wait another minute and see if mail works. If you still have a "self assigned IP address" in your Network control panel, try deleting /Library/Preferences/com.apple.alf.plist and rebooting.
7. If you can't load a web page at all, there may be a cable/DSL problem. (If you use a cable modem, turn on the TV. If there is no picture, then the cable service is down.)
8. If all that fails, call cable/DSL support, navigate prompts, wait. They usually want to know your home phone to look up the account. They will ask "if you have a router." The last three times I've gotten this far, the first level tech was completely clueless, knew nothing about Macs, had me restart the computer and power cycle the cable modem, asked if I could reinstall Windows (I said no), and then scheduled a truck to come in a few days. Then the problem went away after a few hours, and they called to cancel the truck.

Stuck Application

If an application will not respond, won't drop its menus, or you can't quit it, try holding down Command and Option and hitting ESC. Then select the application that won't quit. Killing an application this way will lose all its unsaved changes. It is a good idea to get in the habit of doing a Save often, especially if you are having problems with apps like Microsoft Word. If you can copy the whole document before killing the application, maybe you can save your content.

Restart

Sometimes the Mac operating system gets screwed up. One thing to try is restarting: select  ► Restart. This will quit all applications, reset the hardware, load a fresh copy of the operating system, and reinitialize the OS tables. If your computer is completely frozen and won't restart, hold down the power switch for 10 seconds to force a crash (this loses unsaved work).

Safe Restart

If your computer crashes every time you start it, try to restart it in safe mode. Hold down the shift key and start. Keep holding down the shift key until the desktop is displayed.

Zap the PRAM

Rarely, it will happen that the hardware's Parameter RAM (PRAM) is corrupted. This little bit of memory holds a few values used by the OS while the machine is off. You can "zap" (that is, reset) the PRAM by  restarting while holding down Command-option-p-r. The computer should chime, then chime again. I had to do this when a MacBook Pro wouldn't display anything after it woke from sleep. After you zap the PRAM, the machine will forget how loud you had set the system sounds.

Reset the SMC

Rarely, your computer's System Management Controller will get confused. This chip controls noticing lid opening and closing, display backlighting, display selection, fan speed, and other hardware related functions. Put the machine to sleep and wake it, and do a restart, before resetting the SMC.  Directions for resetting it depend on what kind of Mac you have. For a Mini, you unplug all peripherals and the power, then hold the power button for 10 seconds. For some laptops, you take out the battery.

Older PowerPC Macs have a Power Management Unit (PMU) instead, and the reset procedure is different. Use Google to find procedures for your machine model.

Run Hardware Diagnostics

Hardware problems are very rare, but when they happen, your computer may behave inconsistently and fail in strange ways. It may be a good idea to rule out failing hardware by running the diagnostic programs that came with the machine. Find the original boot disc that came with the computer and put it into the CD slot. Then shut the machine down, and restart it, holding down the D key on the keyboard. (You may need a wired mouse instead of a wireless mouse.) Click the Test button to run the tests. If it says you have a problem, WRITE DOWN exactly what the diagnostic message was.

The Apple diagnostic runs a simple memory test. If you suspect that you have a memory problem, you may want to download and run memtest. There is also a (free) point-and-click interface to memtest called Rember that you can download.

Repair the File System

If you've had disk problems or system crashes, the contents of your file system may be corrupted. Once that happens, your computer will run even worse. Boot the computer from the install disc while holding down the C key, and instead of installing the operating system, choose Utilties ► Disk Utility. Then highlight your boot drive in the left sidebar and click Repair. If this process finds and fixes problems, then something bad happened in the past: your hard drive may be getting ready to fail, so do a backup. Running Repair is always OK to do: it never makes things worse. Sometimes Disk Utility will say it cannot fix your disk; this is big trouble, and you may end up having to wipe your disk, reformat it, and reload from your backup.

Repair Permissions

Every file on the computer has permission settings that say whether the file can be read or written. These permissions sometimes get set wrong, for instance if you have trouble installing or removing an application. A few applications' installers don't set permissions the way Apple thinks they should be. You can run Disk Utility (no need to boot from CD), select your boot drive in the left sidebar, and click Repair Disk Permissions. This will take a while and may produce a list of messages saying it changed something. It's always safe to do this repair, and doing it may help sometimes.

Restore Deleted Files

Before a problem started, did you "clean up a lot of stuff you didn't think you needed?" This is a common cause of difficulties. You should be running Time Machine. If so, you may be able to restore the deleted files.

Reload the Operating System

Boot the computer from the install disc while holding down the C key. The Installer window will come up. Tell it to go ahead, and wait while it reinstalls the OS. Takes an hour or so. Then go into Apple : Software Update and re-apply all the software updates. This process won't delete your personal files or data.

Delete Keychain Entries

For some WiFi problems, you may need to open your system keychain and delete entries for "Airport" and re-create them.

CD won't eject

If your computer won't eject a CD, try

1. run Disk Utility, select the disc and hit Eject.
2. run Terminal and type drutil tray eject.
3. Restart the computer while holding down the mouse button.

CD or DVD problem

If your computer displays and alert box about "failed to calibrate the laser power level" when trying to write a CD or DVD, use the Laser Lens Cleaner and try again. If you insert a CD that has read errors, it may take several minutes of grunching and retrying before the OS gives up on it and ejects it. Run the Lens Cleaner before trying it again.

Wireless Networking Problem

Networking problems are hard to diagnose. If you have trouble with wireless networking, try

• Turn AirPort off and use wired networking, and make sure that works without problems.
• run Keychain Utility and look for passwords associated with the access point you are trying to log in to. If you find one, delete it and see if you can then connect after re-entering the password.
• Some users have cured their network troubles by changing the channel used by their base station.
• Interference from cordless phones or neighbors could cause difficulty connecting or staying connected.
• in  ► System Preferences ► Network, you can select Airport in the left sidebar, and click the minus sign at the bottom of the sidebar. This will delete all the AirPort settings. Then click the plus sign at the bottom of the sidebar, and choose AirPort for the interface, and name it "AirPort". This will discard old settings that may be corrupt or screwed up. You will then have to re-enter settings and passwords.. but sometimes this fixes mysterious problems.
• Some people have cured their network troubles by deleting networking-related preference files and then restarting the computer.
• The (Snow Leopard) tool /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport may help a real wizard find out what is happening on a wireless connection. I have not used it.

RTFM

A manual came with your computer. Read it.

Try searching  Apple's support site for help. In the left hand panel, pick your product from the "all products..." selector, and then look under "Troubleshooting" on the resulting page.

You can also search for help on Google.. but beware, sometimes you find wrong answers there. Same is true of the Apple Support Discussions pages.

Apple Service

When you run out of things to try, call AppleCare. You bought AppleCare with your computer, as I suggested, right? It is essential. (It is not available in all countries though.)

Before you call, find your computer's serial number and make some notes about what your problem is, and what you have tried (see "Reporting a Problem" above). I used to put a sticker on the front of each Mac with its serial number and the phone number of AppleCare. I think Macs are more reliable now: I have called AppleCare about four times in the last ten years.

The time may come when your computer does not work, and the AppleCare specialist says "bring it in to service." If at all possible, take it to a real Apple Store rather than to an "authorized service provider." I had really bad experiences with the supposedly expert service people at CompUSA. Not all non-Apple repair places are bad, but the good ones are hard to find. For some kinds of problems, Apple may arrange for shipping.

• If you can, clone your hard drive to an external disk before you take it in for service.
• Make an appointment first. If you just drop into the store, they will probably ask you to make an appointment and come back later.
• If possible, avoid weekends: the Apple stores I have visited are very busy then.

After A Repair

So, you took your machine in to the Apple Store, and they worked on it and gave it back to you. What next?

If Your Files Are Gone

Your hard drive may have crashed. Apple repair will have installed a new hard drive, and usually re-installed the OS. So this is like having a new machine. The machine will go through the "first boot" sequence when you start it up, and you will have to restore your files from your Time Machine backup, or other backups. If you don't have a backup, you are in big trouble. You can ask Apple to give you the old crashed hard drive, and Google "Drive Savers," and pay thousands of dollars to see if they can recover your data.

If Your Logic Board Was Replaced

In some cases, the "mother board" or "main logic board" has to be replaced to make your computer work again. Your files may be all just fine.. but things are not necessarily perfect. There are two possible pitfalls.

One thing that can go wrong is that the Apple Store techs may have forgotten to transfer your old serial number to the new machine. If this happens, and you have any applications installed that are locked to the CPU serial number, they won't run. Adobe Photoshop and Illustrator behave this way. If this problem occurs, you will have to call Adobe Tech Support for help. (They don't work on weekends.)

Even if the serial number is correctly transferred, you will discover that the machine has a new MAC Address. What's that? It's a number that identifies your network adapter, and no two machines have the same one. This matters because Time Machine uses it to identify backups: so if your MAC address changes, then Time Machine won't see your old backups, and you may have to do some adjustment to get it to start backing your machine up again. The solution to this problem is different for different versions of OS X; search Google for advice if you are in this situation.