It sounds as if you somehow locked the system and when that is done it can only be unlocked by the user who locked it (which is why you see a log on option instead of a log off option and why pushing the logon option gets you only to that one screen because that is the user who implemented the lock. This is normal and the way it is supposed to happen. You can't switch users if the system is locked by another user. Once you login, you should be able to then go to the Start Menu and switch users since the system is no longer locked.
If switch user is not showing up as an option in the Start menu and the problem is not that the system is locked, then it may have become deactivated. Here's a procedure to reactivate the switch user option in the start menu if it is missing:http://www.vistax64.com/tutorials/89939-fast-user-switching.html.
If you want to switch users (and don't mind closing all the windows for the current user), then you can hit ALT-F4 and that will eventually bring up a shutdown window. Click on the arrow next to the chosen option and the other options will appear. One will be Switch User. Select it and click OK. The switch user process should then begin.
I hope this helps.
Good luck!
Lorien - MCSE/MCSA/Network+/A+ --- If this post helps to resolve your issue, please click the "Mark as Answer" or "Helpful" button at the top of this message. By marking a post as Answered, or Helpful you help others find the answer faster.