Ability to hibernate is enabled by default on every Windows Vista installation. If you have disable and turn off hibernation feature, accidentally delete hiberfil.sys using Hibernation File Cleaner of Disk Cleanup tool, or you just simply cannot and unable to find the Hibernate button to put the computer into hibernation state, you can re-enable or try to enable and turn on the hibernation functionality in Vista again with the following command. In Windows Vista, users no longer able to enable or disable hibernation via desktop graphical user interface.
How to Enable and Turn On Hibernation Feature in Vista
1. Click Start button, then type Cmd in the Start Search box.
2. In the search results list, right click on “Cmd” and click on Run as Administrator item on contextual menu.
3. In the command prompt window, type the following command:
powercfg -h on