9/24/2008 -- This is a known problem in Vista. This happens when you run Disk Cleanup Tool and it disables the hibernation file, which needs to be enabled before you can use the hibernation feature or the hybrid sleep feature.

The Disk Cleanup Tool can be quite disastrous in the sense that if it disables hibernation and the hybrid sleep feature is enabled, the backup of open programs is not saved to the disk when you use the sleep feature. Therefore, you can potentially lose some important data. Also, if your computer loses power while in the sleep mode, all unsaved work will be lost.

If your Vista is no longer able to hibernate, run the "powercfg –h on" command at the command prompt as an administrator. This will enable hibernation and the hybrid sleep feature, as explained in KB article 928897.