My mac won't wake up from sleep mode?

Try resetting the SMU as shown in Apple doc.#300341.
If that doesn't do it, post back!

Cheers!
DALE

MaxFS,
The other poster had a big G5 tower. The procedure for an iMac is shown in Apple doc.#301733.
If you still have this issue after the reset, post back.

Cheers!
DALE

Apple doc.#301733 again. Reset the PRAM as they suggest also.

Cheers!
DALE

If your Mac happily goes to sleep but won't wake up--Hold down the power switch to force-reboot your machine and get it running again.
Don't worry-your Mac isn't narcoleptic. Its Open Firmware system or NVRAM (nonvolatile RAM) is likely corrupt, and your Mac can't remember how to wake up.
Restart your Mac and immediately hold down the Command-Option-O-F key combo until you see a somewhat intimidating command-line display that ends with the Open Fimware prompt. Type the following line:

reset-nvram
press the return key on your keyboard
set-defaults
press return
reset-all
press return
Your Mac will reboot. Once it's running, put it back to sleep to test. If the problem recurs your Energy Saver preferences may be the culprit. Delete the com.apple.PowerManagement.plist file (go to Finder/Hard drive/Library/Preferences/SystemConfiguration)--don't worry, your Mac will regenerate a new one--and then restart to see if that rights the wrong.
Still didn't work? Might be time to call AppleCare or see an Apple Genius or a technician.

Cheers!
DALE