Steps to boot in single user mode in Ubuntu 12.04:
Step 1: When you start your system, press "shift" key continuously to get the grub loader screen.
Step 2: In Grub 2 menu, select the menu with Linux 184.108.40.206-generic-pae highlighted.
Step3: Press 'e' to edit the grub2 menu.
Step 4: Move the cursor to the line that starts with "linux /boot/vmlinuz-3.2.0-23-generice-pae".
Step 5: Change the content "ro quiet spalsh $vt_handoff" To "rw init=/bin/bash".
Step 6: Press "Ctrl+x" to continue boot to in single user mode.
Step 7: Now you will get prompt of the root user.
Step 8: Change root user password,
# passwd root
Step 9: Now sync and reboot the system i.e.
# reboot -f
I hope, above steps are helpful to change the root user password in single user mode.