Hibernate option is disabled by default in Ubuntu 12.04. Hibernate doesn't work in many cases with Ubuntu, so this decision was taken to avoid data loss/corruption that can happen when a user tries to hibernate his system.
So if you are quite used to hibernate feature, you will have to rely on a workaround by manually editing some system files. There is no GUI option to enable/disable hibernate in Precise.
This default hibernation policy affects Kubuntu and Xubuntu as well and a bug has been filed to at least allow enabling and disabling hibernation from system settings. However, at the moment it has been marked as wishlist.
Personally I haven't used hibernate option in a long time but I know it is important for many users. If you want to manually enable hibernation in Ubuntu 12.04, follow the instructions below.
First, test hibernation works on your system by running command:
If everything is fine, you can continue to use this command to hibernate your system.
You can also add hibernate option in menus. Run the command below (tested with Ubuntu only, may not work in Kubuntu/Xubuntu):
sudo gedit /etc/polkit-1/localauthority/50-local.d/com.ubuntu.desktop.pkla
Add the following to the file and save:
Restart your system to get hibernation back.