ITunes acting funny PLEASE HELP!!!?

So last night i was trying to sync my iphone and itunes didn't see it (when i pluged it in it charged but didn't show up in iTunes. So i closed out of it and when i tried to turn iTunes back on it gives me the message "Itunes cannot run because some of its required files are missing. please reinstall iTunes" what do i do so that i dont lose all of the things i have in iTunes? PLEASE HELP!!!