If the hiberfil.sys file on the system partition is removed, the option to hibernate is not available. A common reason for the hiberfil.sys to be deleted is from running the Disk Cleanup Tool, as explained in the Microsoft article "The hybrid sleep feature and the hibernation feature in Windows Vista may become unavailable after you use the Disk Cleanup Tool" (http://support.microsoft.com/kb/928897/en-us ).
To create the hibernate file and reenable hibernation, use the following command:
powercfg -h on
In Windows Vista, run the powercfg -h on command at a command prompt to enable the hibernate feature and the hybrid sleep feature.
