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

Symptoms:
If some or all of the keys on your Apple keyboard don't seem to be working, use these tips to troubleshoot the issue.

Note: If you use Mac OS X v10.4.x and an external Apple Aluminum Keyboard, you may need to download and install certain updates. Mac OS X v10.4.11 or later is recommended.

Products Affected
Keyboards, Mac OS X 10.4, Mac OS X 10.5

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.)