I find I can reboot even the most resistant machines by pulling the AC cord and on a laptop the battery. Then on bootup I can login as myself or administrator if I need to do some admin work on my PC.
Before you pull the AC cord try the usual power off by tapping the power button. If that didn't do it, press and hold the power button for 10 to 20 seconds.
I have an Acer Laptop(8104WLMI), original software and when it starts up I get:
"This computer is in use and has been locked.
Only ACER-57DBBDB911\Don Claffey or an administrator can unlock this computer"
Title of Dialog box ? "Unlock Computer"
The cursor is in box titled User name and I cannot type anything in the box nor can I turn the computer off.