iMac won't "wake-up" when put to sleep

I went to the Apple store, explained this issue and received the following feedback:

This happens because the keyboard also falls asleep. To wake the keyboard, press the space bar, then the ESC key should wake the computer.

This did not work for me. I also tried to connect a USB keyboard from my Mac Mini and press ESC on that. This also did not work. 😟 This leads me to believe that it is not an issue with the keyboard, but rather with the OS.

You may find the solution in the

iMac won't wake up from sleep using keyboard or mouse thread.

My Mac won't wake at all without disconnecting from the mains (every time not just once) I am told it is a firmware problem which Apple is aware of with some computers and are working on a fix

everyone having this problem, please examine your area where the MAC device is located. I read on this thread where a person had a problem with their iMac running hot and that was their cause for not waking after sleep. after investigating, my mac was definitely hot to the touch at the back & top area and it was sitting pushed back as far as it could go on my desk where I guess it was not getting enough ventilation. I have since moved it out 2 inches so the top vent is not covered and I'm now going on 4 days with no problems waking from sleep. before I had to power it off/on 3-4 time per day. it worked for me so please investigate a possible over heating issue before restoring your hard drives and replacing your devices.

good luck and hopefully this simple solution works for many of you.

it did lock up on me once again, but this time I relized itunes running in the background (for apple tv streaming videos and music, etc. I stopped itunes from running and so far it's been almost a week without issue of not waking from sleep...