Note the following is from http://www.askvg.com/how-to-re-enable-missing-hibernate-option-in-windows-vista/
Actually its a known issue in Vista. The hibernate option disappears whenever you run "Disk Cleanup" because Disk Cleanup wizard also removes Hibernate files. So if you use Hibernate then make sure you deselect "Hibernattion File Cleaner" option from the list in future.
Anyway its very easy to re-enable missing Hibernate option by following these simple steps:
1. Click on "Start button -> All Programs -> Accessories". Right-click on "Command Prompt" and select "Run As Administrator". If you are prompted to enter password, enter the password and continue. You can also open Command Prompt in Administrator mode by typing "cmd" in Startmenu Search box and press "Ctrl+Shift+Enter".
2. Now provide any of following commands:
powercfg /hibernate on
powercfg -h on
3. Exit command prompt and Hibernate option should appear in Startmenu.