I have found this answer on another website...
Kris;
This is common with LCD monitors, as they age the matrix can fail. This is not a problem with the LCD it's self, but the circuit that drives it. Most units fail like this after six or seven years. I would recommend contacting dell, since the unit may still be under warranty. If not you may be lucky and have a repair center in your area, that will handle a LCD unit. If so the repair should run around $75.00 to $90.00. At that price you might consider the repair, over that I would replace the unit, since you can get the same size or larger for around $200.00 in LCD and $175.00 in a CRT monitor.
Good luck;
Phill
Neil Mitchell
December 2007