In Ubuntu 12.04 operating system by default hibernate is disabled.
To enable hibernate follow below procedure:
Open /var/lib/polkit-1/localauthority/50-local.d/hibernate.pkla with your favorite editor (vim/nano/gedit) and add following content in it.
[Re-enable Hibernate] Identity=unix-user:* Action=org.freedesktop.upower.hibernate ResultActive=yes
Now save and exit from your editor and reboot your desktop/laptop to view hibernate option.