I have iMac 2011 model which has worked fine until today. iMacs own screen posts no picture at all - only black screen. I have external Apple LED Cinema Display connected which works without problem. I can actually use the computer with external display as normal except iMac itself doesn't post any video. It seems iMac itself and all connected devices work. iMac just doesn't show any video. Unplugging DisplayPort cable also does not make picture appear on iMac screen. When I go to Settings > Displays only LED Cinema Display is shown. It seems like iMac doesn't even recognise its own display. Any suggestions? Already tried PRAM reset.