I use a wired keyboard with numeric pad (came with early 2008 imac 20.5 inch). For some strange reason, it stopped working, sort of. Most of the keys won't have any effect on OS. No letters, no shortcuts, no numbers, nothing.

HOWEVER, the f8~f12 works for the special feature, thus I can still change volume, or start/stop music. Furthermore, with keyboard viewer on, I see the keys have input, but nothing shows up (as in, if I press "a", the "a" key on the keyboard would darken, meaning the key was registered, but "a" would not appear to be have any effect).

Cap lights up, keyboard fully works when attached to another computer, mouse connected to the keyboard works. This indicates it is not the keyboard's problem, but some software issue (something got disabled or enabled). Restarting the computer would make the keyboard work fine.