Just to jot down some notes about this strange problem that happens from time to time in my laptop running Windows Vista – for people who might find the suggestion using search engines from here, and more so for me to look back when it happens the next time.
How it happens
Among other reasons (which I am not so much aware of) you may find that the Hibernate option that was there in your Start Menu has suddenly disappeared after you did a Disk Cleanup activity from Accessories > System Tools – if you had chosen to delete off old “hibernate data files” (or something which will read like that) among other stuff that you decided to remove.
How to get it back
1. Open a Command Prompt window from Start Menu > Accessories using Administrative privileges (“Run as Administrator” option using right click on the Command Prompt menu).
2. Type powercfg.exe /hibernate on
3. The prompt should be back without any output. The Hibernate option should be back in your Start Menu now.
4. Wait for the mythical Vista Service Pack 1 to arrive to fix the problem permanently.