If change username does not appear as an option in the start menu and the problem is not that the system is locked, then it may have become disabled. Once you login, you should be able to then go to the Start Menu and change user because the system is locked is no longer. You can not change user if the system is locked by another user.
It seems that if you somehow locked the system and when it is done it can only be unlocked by the user who has locked (which is why you see a log on option instead of a disconnect option and pushing the logon option gets you only to this one screen because it is the user who put in place the lock.) This is normal and the way it's supposed to happen.
