I was going through all the Vista power options, trying to get my Hibernation mode back on, which seemed to have disappeared.
I found out the hibernate option will disappear if you have recently run Disk Cleanup, which I did. By running this it removes the hiberfil.sys file resulting in no hibernation. See Microsoft article KB928897.
After googling around, I found a simple method to enable / disable hibernation:
1. Click start, type “command” in the Start Search box (without quotes)
2. Right-click Command Prompt in the Programs list, and click “Run as Administrator”
3. If User Account Control prompts you to allow the action, click on Continue.
4. In the Command Prompt, type “powercfg –h off” (or “on” without the quotes), then press enter.
5. Close the Command Prompt window and you’re done
You should now have hibernate and hybrid sleep options for your power profiles.