I had enabled "hibernate" option in my Ubuntu 12.10 few months back by adding the following lines to
[Re-enable hibernate by default] Identity=unix-user:* Action=org.freedesktop.upower.hibernate ResultActive=yes
However, I don't know how, rather don't remember what led to it being disabled both in "power options" and in the "gear icon" on the menu bar.
The file is still intact.
One of the things I suspect is removing old kernels using Ubuntu tweak, but have no valid reason to allege this :P