The problem can be the display, the graphics adapter or the mother board. If you connect an external monitor and it shows your desk top then you have a bad display. If you don't have an external monitor, use a flashlight angled to see if you can see icons on the desktop, this narrows the problem to a back light on the display. If none of these steps work, then you may have a bad mother board or bad graphics adapter. Shut down and remove all the memory and see if you get a beep error, a beep error will indicate that the mother board is working.