Problem with Uplay (Assassin's Creed 3)

I was playing suddenly the computer froze, restarted the computer and Assassins Creed 3 does not start. I entered my data into Uplay, but when it was not my nickname, but some kind of guest-AJfLhSgb. and there are no uplay games. And when I start AC 3 I asks for a digital key, and he writes the key is already in use by another Uplay account. HELP ME PLEASE