Visual Editor icons disappeared from our screen, and the “Text” tab would not work. We were stuck in “Visual,” with no icons, and all of our content had a white font and was therefore invisible except when “selected.” Two of us Admins — far apart and using different browsers — experienced it.
I followed the debugging procedure on http://codex.wordpress.org/Using_Your_Browser_to_Diagnose_JavaScript_Errors
It worked! We refreshed our screens, and there were the icons.