This must be something simple, but I was using my machine, walked away for a few minutes, came back to a blank screen, light still on. Could not get screen to reappear. Shut off machine (button on back) unplugged it from the wall, plugged back in and tried to turn it on, but nothing happens. No chime, no gray screen. Light comes on but nothing else happens. Tried unplugging and turning on and using the command, option P and R....nothing happened. Now I am stuck as to what to do....anyone have any suggestions?