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.
P.S.: One of the things I suspect is removing old kernels using Ubuntu tweek, but have no valid reason to allege this :P