Some or all keys on the keyboard don't work (Mac OS X v10.4, 10.5 or later)

Resolution

No keys work (external keyboard)

• If you use a wireless Bluetooth keyboard, make sure it is turned on when the computer starts up. You may need to check the battery levels, too. See this article for more troubleshooting information.
• Disconnect and reconnect your keyboard if it is external. Make sure that the connector is completely inserted into the port. (USB plugs fit into ports one way only, and won't allow you to make a connection if you try to connect them in upside-down.) Test the keyboard again.
• Connect your keyboard to a different USB port.
• Try a different keyboard on your computer if possible, or your keyboard on a different Mac.
• Try logging into a different user account. If the issue does not occur when logged into a different user account, then troubleshoot for software issues.
• Open Universal Access preferences (in System Preferences) and click the Mouse tab. Verify that Mouse Keys is Off, as shown below. (With Mouse Keys on you can use the keyboard or the numeric keypad keys to move the mouse, however normal keyboard functionality will be disabled until the Mouse Keys feature is turned off.)

No keys work (built-in keyboard)

See Mac OS X: Low battery alert does not appear over full-screen application, keyboard becomes unresponsive.

MacBook / MacBook Pro built-in keyboard only produces numbers

• Make sure the Num Lock key is not active (press fn-F6 to toggle Num Lock).

Eject key or Caps Lock key doesn't seem to work

See these articles:

• Apple Keyboard (2007): Caps Lock modified to reduce accidental activation
• Media Eject key delay

Some keys don't work as expected

1. Open Speech preferences in System Preferences. If "Speak selected text when the key is pressed" is enabled, the key combination to speak text cannot be used for other purposes or used to type text--change to a more obscure key combination (try to use more modifier keys such as Shift, Command, Option, and Control). Or, simply disable the "Speak selected text when the key is pressed" option.
2. Open Universal Access preferences in System Preferences, click the Keyboard tab, and make sure that Slow Keys is turned off. With Slow Keys on, you need to press a key for a longer period of time for it to be recognized.
3. In Universal Access preferences, click the Mouse tab, and make sure Mouse Keys is turned off. With Mouse Keys enabled, you cannot use the Numeric Keypad to enter numbers--instead the keypad moves the pointer (cursor). (There is an option to enable Mouse Keys with five presses of the Option key; you may want to turn that option off to avoid accidentally enabling it.) If Mouse Keys is enabled and you are using a keyboard with no numeric keypad or Num Lock function, see Unable to type while Mouse Keys is enabled in Mac OS X.
4. If the function keys on the top row of the keyboard are not working as expected, see Mac OS X 10.4, 10.5: How to change the behavior of function keys.
5. If the issue persists, open System Preferences and click International.
6. Click the Input Menu tab.
7. Click the Keyboard Viewer "On" checkbox to select it.
8. From the Input (flag) menu, choose Show Keyboard Viewer.
   
    
9. If the keyboard is connected and detected by Mac OS X, the keys you type will highlight in the Keyboard Viewer window. Try typing to see which keys are not highlighting in Keyboard Viewer.
10. Open TextEdit (or another text application) and try to type something using the keys that were previously not responding.
11. For Mac OS X 10.4 or later, start from the Mac OS X Install Disc, choose Terminal from the Utilities menu and test the keys which were previously not working.  If the keys work while started from the Install disc, then the keyboard itself is working correctly.  Use Mac OS X: How to troubleshoot a software issue to isolate the software issue that may be causing the keys to not respond.