if that doesn't work just delete preferences. Hapens to me sometimes, this allways works for me.

My doccuments>maya> 4.5 (version that you have)>prefs> then delete user Prefs. a new one will pop up when you restart Maya.

Hope this helps!
Brian

All else fails, delete prefs, it fixes all kinds of stuff, if stuff starts disapearing. almost like Ducktape.