IMac screen is black and won't turn on

Question

I got a 27" iMac (intel) a couple months ago (mid-year 2010) and sometimes the screen won't start up after it goes to sleep. Most of the time, I will leave the computer and the screensaver will start then the display will turn off and go black. When I return and hit a button on the bluetooth keyboard or move the Magic Mouse around, the computer will come out of sleep, display will turn on, and everything works the way it's supposed to. However, every now and then, the screen will go to sleep (turn black) and it won't turn back on when I hit the keyboard and move the mouse. I can just barely hear the fan still working, so I know the computer is on but asleep. Even when I hit the power button or hold it nothing happens, it won't even restart. I have to physically unplug the power cord and replug it back in then hit the power button to get the computer to start up again. I have moved the computer around to different outlets and even tried it at a different house, but this issue still occurs. Am I doing something wrong with the Energy Saver settings or is there just something wrong with my computer?
Thanks!

Answer 1

Hi, I hope you find this information useful:
A few years ago, we purchased iMac 27" 11,3 (Mid-2010) (with i7 2.93Ghz, ATI 5750 1GB VidRam, etc.) for our offices and software development team. They were originally shipped with Snow Leopard and worked as expected and without any problems. However, a year later, we upgraded to Lion and all of machines immediately exhibited the same problem: the screen would go to sleep by itself and would require a hard reboot or sleep on/off. (Only the screen was asleep and the rest of the iMac continued to function; this was confirmed by screen sharing to the machine.)
Due to the severity and impact on our business, we immediately contacted Apple and their genius bar staff in New York/SoHo. They were unable to determine the root cause, so they replaced all of the logic boards, video cards, and screens, and got us back online asap. This resolved the problem, but we were curious to know why. After days of rigorous testing, we concluded that the issue is caused by a bug in MacOS. Specifically, faulty Kext files (Kernel Extension files extend the functionality of the MacOS Kernel) for the ATI drivers.  So, why did the replacing the hardware work? We think that the Kext files that were released and distributed with Lion (and later MacOS versions) correctly supported the newer hardware revision of the replacement components, but not the older ones.
If you're experiencing this problem, short of replacing the components or waiting (for another 3+ years) for Apple to hopefully fix their OS software, you can lessen the effects and be able to recover faster by doing the following: (Please bear in mind that these are not permanent fixes.)
Turn on "Hot Corners" and assign one corner to "Put Display To Sleep". When your screen goes to sleep, move your mouse (which still works) to that assigned screen corner for a second, then move it again. This will wake your screen. You may have to do this a couple of times, but it's much faster than a hard reboot or a sleep on/off for your iMac.
Turn the brightness of your screen to a low setting (10%-15% of the total brightness).
Turn on screen sharing, so you have a last resort method of accessing the machine.
If you're not convinced that it's a MacOS software problem, then do what we did and see for yourself; install Windows 7, Windows 8, or Linux via Bootcamp or as the only OS on the iMac. Those operating systems and drivers worked flawlessly for us; the bug does not occur and the screen does not go to sleep, but if you dual boot and switch back to MacOS, the bug reappears because the iMac is loading faulty Apple MacOS software.
Good luck!