Hello, My mate has a 2011 iMac. The screen will boot up for perhaps only 2 secs when powered up via a normal short press of the power button. A long press triggers the fans / cpu but the screen remains black for circa 40 secs, then it quickly turns white for two seconds and shuts down again. Any advice please? I have asked him to check his power lead, it is connected directly to the wall socket and the input is securely in the iMac.