What monitor do you have, you may have to reset the monitor. For instance on our Planar monitors at work:
- There are 5 buttons on the front. Unplug it, then hold down buttons 2 & 3 (left to right) and plug it back in. The light will flash yellow and green. When the light is solid green, let go of the buttons. If you see a blue box on the screen that says something like "auto adjusting" the monitor is working again. :You will probably have to do this many times to get it to work.
Your back light may have burnt out (flat panel) turn off the lights in the room can you barely see a display...?