I shut down the computers over the weekend (usually keep them running, but trying to save energy). Two of them rebooted fine....the third one, which is the main computer on the network shows a blank screen. CPU lights are lit, fan running, monitor's green light is lit. I tried hitting keys, moving the mouse, but still black screen with nothing. I've turned computer off, disconnected plug, replugged and restarted several times. Nothing works. The main computer does not show up on the network. All are running Windows XP. Any ideas?