iMac won't sleep or sleep hard disks either!

My iMac is late 2009 and am running Mountain Lion 10.8.2. I used to be able to click sleep under Apple menu>Sleep or by simply pressing the sleep button at the back of the iMac to ensure it goes to sleep including the hard disks. However, for the past 3 days the screen will just go black but not sleep, as if I'm pressing a command to turn the screen off only.

I checked to make sure bluetooth devices do not wake up iMac.

I checked energy saver to make sure that 'Put hard disks to sleep when possible' is checked

I checked that network interuptions will not cause iMac to wake from sleep

I checked to make sure downloads or programs are not running in the backround to prevent the iMac from sleeping.

I checked to make sure that USB devices are unplugged and even ethernet cable.

I checked other forums and common answers to this problem would be the above I listed which I made sure is not my problem.

Please help my iMac's insomnia!