Alright, been having this problem for about 2 weeks. (27 inch iMac from 2010) Everytime I restart my computer, or shut it down, it restarts with a completely black screen. It'll play the chime, it'll log in and reopen programs, I can tell that stuff is running on my computer, and I can even open itunes and play it with the buttons on my keyboard. But, the screen is black. I've tried resetting the PRAM to no avail. I've disconnected my wireless keyboard and mouse because I read somewhere that helped. I've unplugged it, plugged it back in, and it doesn't work exactly. It seems to work/fix itself when ever it feels like it. One time, the disconnecting of my wireless devices worked (or it was just an odd coincidence). Another time, unplugging the iMac for a good 2 hours and plugging it back in worked. I have no idea what I'm doing. Help, please? I can't restart my computer without it having this issue.