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

so you can't even wake it up by shortly pressing the power button?
check if the following are activated:
system preferences -> energy saver -> "allow power button to put the computer to sleep"
system preferences -> bluetooth -> advanced -> "allows bluetooth devices to wake this computer"

if both are checked, then it might be a third party application doing this. do you have any applications that might be the cause? you might also wanna check the "Console" application and see if it reports anything useful for the time you send your computer to sleep.

Someone else here mentioned a multi-port USB device being plugged in. I had exactly this problem and after taking out the 7-port USB everything is fine again.!!

I am having the same intermittent problem. Sometimes wake up from Bluetooth keyboard / mouse works, sometime it doesn't. When it doesn't, even a wired USB mouse won't wake it. Power button doesn't wake it. Nothing. Only power button long-pressn to turn off and reboot works. Frustrating.

I just replied to a 14 page thread regarding the same problem. It seems there is no one solution for this. My guess is software related, but who knows. I've done everything from resetting PRAM/SMC to reinstalling wireless keyboard/mouse/trackpad, to repair disks, etc. I will reload Snow Leopard OS just to see if that works. Im out of options other than going back to Apple.

This has been happening to my iMac occasionally as well. Can't wake the system with my wireless apple keyboard.

My story is very like photo-music-guy: sometimes it wakes from sleep, sometimes it has to be restarted. It will happen (or not happen) overnight or after a lunch break; so far not from a 5 minute nap. The 27" iMac people have a couple of long threads going about a similar but more serious problem - their machine will suddenly go to sleep (or crash) in the middle of working, sometimes 3 times a day - but we may hope that ours is but a different small glitch.
What I have to offer is mostly negative: machine is new (iMac 11.2 bought January 2011, 4 GB memory), with no peripherals connected yet (oops, one - a Wacom tablet) and only Office for Mac and Adobe CS5 loaded. And the wireless keyboard & mouse, which is sort of the crux of the problem: when everything is either 'asleep' or 'off', hitting the keyboard doesn't do anything AND there is no way to tell if the message got thru or the keyboard is at fault. A blank screen is a blank screen. Holding down the power button for a looong time seems to be the safest way to get a restart & get on with things; very tedious.
Thank you Nachdenki, I did check the powersave and bluetooth settings, and they were both as you suggested.

Hi,

same problem here. Won't wake up using bluetooth keyboard or trackpad. doesn't happen everyday, but annoying when it does.
Otherwise, very very very pleased with the iMac! ten times less stress compared to all the microsoft os i've used in the past.

jw

I am having the same problem. New iMac. A couple of times a week, it won't wake up with the keyboard or mouse. The only solution is the long power button hold. HELP!

How many people with this problem are using the bluetooth keyboard & mouse, raise your hands!
I'm still getting the 'turn-off' now & then, mostly after sleeping overnight. But with my Wacom draw pad connected i have a power indicator light on the tablet to check, so i know if the Mac is asleep or has turned itself off. In the latter case holding down the power button for 15 to 30 seconds brings it back up, "bong" and all. While this doesn't fix the problem, knowing what is going on makes it more of an annoyance and less of a worry, if that makes sense.

Me too. My solution is to unplug the power cord and plug it back in. Works immediately but doesn't explain why this is happening. Something Apple needs to address if this is a common occurrence. And yes I am using the wireless keyboard and mouses with my brand-new iMac.

I've seen this problem as well, but it has manifested in a few other ways.

1. Will not wake up
2. Sometimes other parts are on while "sleeping" (i.e. case or cpu fans)
3. Will wake up, but Keyboard and Mouse will not operate after a few seconds - Keyboard/Mouse connected to USB Hub.
4. Other USB devices hooked will not work through the USB Hub. Devices still connected to the iMac still work.
5. Saw this issue develop with 10.6.5 and afterwards. Did not see on prior releases or on 10.4 or 10.5.

I have isolated the problem on my system to the USB Hub (Belkin). I fixed many of my problems when I moved the Apple Keyboard/Mouse to the iMac. My belief the OS is messing this up and not the Hub. It could be to the I/O Controller Hub within the machine since if I unplug the Hub from the iMac and connect another device the system will not respond. When it is locked up, the only fix is to power cycle with the on/off switch for four seconds or so and reboot the system.

The other users that are seeing the same issue have USB Hubs connected?