This problem occurs when the Disk Cleanup Tool disables the hibernation file. The hibernation file must be enabled to access the hybrid sleep feature and the hibernation feature in Windows Vista.

When the hibernation file is disabled, and the hybrid sleep feature is enabled, a backup of open programs and open files will not be saved to the disk when you use the sleep feature in Windows Vista. Additionally, if the computer loses power while the Windows is in sleep mode, open programs and open files will not be recovered, and any unsaved work will be lost.

Also You cannot see the Hibernate option in Power Options.

Solution

access the command prompt and type the text below to turn hibernation back on.

powercfg -h on