Hmm, well try looking in the Spell Book. (Just hit P on your keyboard to bring it up, if you haven't changed anything.) After that, check your Interface settings to see if you did something to it. (Esc button, Interface) Check to see if an addon is doing it, research it and it's potential problems on the net. (Curse.com, etc. Things like Bongos, Bartender would do it. Disable 1 addon at a time and try to find the culprit, or turn them all off.)
Finally, maybe ticket a GM. Might be a bug.