DEll Latitude D600

The back lighting for the LCD panel has gone out. The picture is there, you just can't see it without the backlighting. So, it could be the Inverter that supplies power to the back lights or it could be the CCFT lamps that burned out. In either case your local computer repair shop should be able to fix it. If it is still under warranty take it back to where you bought it and they should have it repaired under warranty.