Computer not waking properly from sleep mode?

I have a computer problem. Whenever my computer goes to sleep (whether it be me putting it to sleep from the start menu or whether it sleeps itself after inactivity), it becomes unable to wake properly.

Traditionally, I would sleep it then press keys on the keyboard and it would wake itself, now I have to press the power switch on the case itself. In addition, pressing this switch does not truly wake it from sleep mode, i get told that my computer had crashed and it does not save my stuff from when i slept the computer. It just starts normally and I have to reload all windows that I had up at the time.

How can I fix sleep mode?

Sleep mode was working b4 I reformatted my computer about a week ago. (I reformatted to replace hard drive.)