Has hibernation been missing from vista which was available earlier.
Here is what you might have done to make it disappear. If you use the disk cleanup to clean your hard disk and selected the hibernation file cleaner in the disk cleanup. The hibernation option disappears. I think its a bug.
Here is how to fix it. click windows key + r and in the run command type cmd.exe and click ok. Then type powercfg -h as command at a command prompt to get back the hibernate.