If the monitor is indicating signal (ie it doesn't go into standby mode) and as you say, you can still identify the desktop image by the reflection from ambient light, then the backlight in the monitor is shutting off. This could be caused by a faulty backlight lamp, or an inverter fault. Either way, you'll need to get the monitor repaired, if it's out of its warranty period you'll need to consider if it's justified or whether it's better economics to put the likely repair cost towards a replacement monitor.
Example of possible replacement cost:http://h10010.www1.hp.com/wwpc/us/en/sm/WF...67-3546596.html
Edited by Platypus, 11 January 2009 - 05:38 PM.