You have been logged off Active Directory, it sounds like. Simply log off your PC and log back on.
Since you logged on to your PC normally, you will not be prevented access to it until or unless your password-locked screen saver engages or you log off. In other words, you are still logged onto your machine but logged off the domain.
If you cannot log back on, your account has been locked. Get it unlocked and log back in normally. If you password has expired, put in a new password. Everything should work normally after you log back in.