Follow these steps to boot in Single User mode in Ubuntu 12.04:

Step 1:   Reboot the server and press “shift” key continuously to get the grub loader screen.

Step 2:   In the Grub2 menu, select the first line with Linux x.x.x.x-generic.

Step 3:   Press ‘e’ to edit the grub2 menu.

Step 4:   Move the cursor to the line that starts with “linux /vmlinuz-x.x.x-x-generic…ro”.

Step 5:   Change the content “ro” To “rw init=/bin/bash”.

Step 6:   Press “Ctrl+x” to continue boot to in Single User mode.

Step 7:   You should now see the root user prompt.