My iMac has just started having this problem. The screen will shut off about 10 minutes after rebooting. The system is still on because I can still access the drives from my laptop, but nothing will reactivate the screen. As far as I'm aware this problem doesn't coincide with any change I made to the system, definitely no new software or hardware. I thought for a while it might be related to display mirroring, so I turned that off, but no luck. Anyone have any thoughts? If I have to take it to a retailer and get it repaired I will, but I'm not even sure what they would do with it at this point. Any help would be much appreciated.