I would tend to guess that someone got a hold of your login information and by logging in, they reactivated your account. They may also have changed your password so that you can't get in yourself.
Did you ever give your password to anyone? Or did you use a password related to something like your birthday that someone you know could figure out?
You might try clicking "forgot password" so that it sends you a confirmation email with instructions to reestablish your login username and password. The only problem is that if you were hacked, they may have changed your email address, too. In that case, you would never get the confirmation email.