Windows enhanced feature

I administer a Windows2000 server.

Every once in a while, maybe once a week, the worker will turn on the monitor and the screen will be totally blank, (blue) just like what it looks like after you just hit enter on your login name and password.
Pressing cntrl-alt-del doesn’t do any thing. The only thing to do is to reset the server. The backup the night before started but never finished.

This is the only clue I can find. The event viewer shows the system as working perfectly. (relatively speaking).

Has anyone experienced a problem like this?