How long ago did this happen?  It sounds like the display driver needs to be
reinstalled.  This is the driver for your video card - not the monitor.

Have files recently been deleted?  What happened since it was working
correctly?  Anything added or removed - hardware or software?

The first thing that I would try if it's only been a few days is to restore
the registry from a date just before the problem occurred using Scanreg
/restore.  This way if all the files are still there but there was a problem
with the registry for some reason, this should fix it.  Anything that you
installed since the restore date will have to be reinstalled.  The
instructions are in this article:

How to Manually Restore the Windows 98/Me Registry
http://support.microsoft.com/?kbid=221512

If that doesn't fix the problem, then you will want to take a look at this
article.  Since this has worked in the past, I would go straight to the
section toward the end "Unable to Adjust Display Settings"

Troubleshooting video problems in Windows
http://support.microsoft.com/?kbid=127139

If this fails, check with the manufacturer of your computer to see if there
are updated drivers.  Your system should have shipped with drivers and it
should be fine to reinstall these if you're still running the same version
of Windows that shipped on your system, but it's always good to see if there
are more recent drivers available.  Your computer documentation should tell
you what kind of display adapter you have.

PattyL