I've reset NVRAM (pram) and tried removing power cable. The only way to 'wake' the machine is to hold the power button down to shut down and then restart. I have the default setting set for sleep. My Mac Pro (early 2009 2.66 GHz Quad-Core Intel Xeon) won't wake from deep sleep after installing OS X 10.8.2.

If the problem still persists and you are running OS X Mountain Lion version 10.8.2, there may be a sleep bug that is effecting the reliability of sleep on a lot of Macs as discussed Problem with waking from sleep with 10.8.2

Wait five seconds, then press the power button to turn on the computer.

Resetting the SMC for Mac Pro, Intel-based iMac, Intel-based Mac mini, or Intel-based Xserve See if the sleep problem still occurs, if it does try resetting the SMC as noted Intel-based Macs: Resetting the System Management Controller (SMC). Hold the keys down until the computer restarts and you hear the startup sound for the second time. Press and hold the Command-Option-P-R keys before the gray screen appears.You will need to hold these keys down simultaneously in step 4. Locate the following keys on the keyboard: Command (⌘), Option, P, and R.Try resetting the NVRAM / PRAM as noted About NVRAM and PRAM

Mac mini very slow to wake from sleep