You could try a System Restore (if you enabled it) and go back a few days or a week and see if that helps. If so then enabled the built-in Administrator account so in the future if something like this happens, you can boot to Safe Mode (F8 at boot), login as Administrator then you can remove or reset passwords as needed.
Open a Elevated Command Prompt
To enable: Enter net user administrator /active:yes
To disable: Enter net user administrator /active:no
How to open a Elevated Command Prompt