To make hibernation unavailable, follow these steps:

  1. Click Start, and then type cmd in the Start Search box.
  2. In the search results list, right-click Command Prompt, and then click Run as Administrator.
  3. When you are prompted by User Account Control, click Continue.
  4. At the command prompt, type powercfg.exe /hibernate off, and then press ENTER.
  5. Type exit and then press ENTER to close the Command Prompt window.

To make hibernation available, follow these steps:

  1. Click Start, and then type cmd in the Start Search box.
  2. In the search results list, right-click Command Prompt, and then click Run as Administrator.
  3. When you are prompted by User Account Control, click Continue.
  4. At the command prompt, type powercfg.exe /hibernate on, and then press ENTER.
  5. Type exit and then press ENTER to close the Command Prompt window.

http://support.microsoft.com/kb/920730