Hibernation is disabled on Ubuntu 12.04 and Mint 13 by default - you can only access it with the command line. To have it as an option in the shutdown menu again, open a terminal and enter the following:
sudo gedit /etc/polkit-1/localauthority/50-local.d/com.ubuntu.enable-hibernate.pkla
By that you create a new text file. Paste the following into it:
Save the file afterwards and reboot. Hibernation should now be an option in the shutdown menu again.