Verify that Mouse Keys is Off, as shown below. Open Universal Access preferences (in System Preferences) and click the Mouse tab. If the issue does not occur when logged into a different user account, then troubleshoot for software issues. Try logging into a different user account. Try a different keyboard with your computer, or your keyboard with a different Mac. Disconnect and reconnect your keyboard, or try to connect it to a different USB port. Read the full Apple resolution here or try out our simple instructions below at first. Mac OS X v10.4.11 or later is recommended. Keyboards affected when running Mac OS X 10.4, 10.5 or 10.6

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. If some or all of the keys on your Apple keyboard don't seem to be working, use these tips from Apple to troubleshoot the issue. Why does some or all keys on my Apple keyboard not work? (Mac OS X v10.4, 10.5 or later)