Go to the options menu, an check the "Login as" list. Maybe your account is missing from there. Launch the game as "None/Manage accounts", and there is an option like retrieve or revive account, I don't know. That might help. I had problems like this once, while I was playing, my laptop was unplugged, and the next time I wanted to join, my account was missing, but I got it back in the Manage accounts menu.