you have three choices there:
1. format your computer, re-install your entire hard drive, install World of Warcraft from scratch, discover the problem again after a month and figuring it was worth reading on.
2. typing
in your chat box, and press enter/console reloadui
this should refresh your entire UI and should fix it.
3. if step 2 doesn't work, log off, log in (though most probably won't be needed)
if the problem persists then you should try a game repair.