Keypad on Apple Aluminium Keyboard not working or seems to have crashed
Thanks for the tip about Mouse Keys. I thought my Mac had developed a software bug that prevented me from using the keypad. It almost seemed like the Mac was crashing when you tried to use it, as the Mouse click also stopped working.
In System Preferences, just go to Universal Access and select the Mouse tab.
Make sure Mouse Keys is set to off, and uncheck the "Press the option key 5 times to turn mouse keys off".
I must have inadvertently clicked Option 5 times yesterday and activated it! Drove me insane for hours!