I'm guessing that your video settings may have tweaked themselves out of the monitor's acceptable range, try this:
Reboot the computer, and right after the POST screen and just as Windows starts to load, hit the F8 key. When the boot select menu opens, select "Enable VGA mode" from the list using the up/down arrow keys and hit enter.
This will start Windows XP in 640x480 16bit @ 60Hz, which almost any monitor out there can handle. It's like a fallback mode that comes in handy when your main monitor dies and you have to go to your 15" backup monitor that's sitting in the closet