How to recover root password

Ref: http://ubuntuforums.org/archive/index.php/t-1177638.html

...
in the grub menu highlight the Ubuntu menu entry (NOT the recovery
mode)

hit e for edit, then highlight the kernel line and hit e again

remove ro quiet splash from the end of the line and

append it with rw init=/bin/bash and hit Enter then b to boot.

type:

passwd username

to change the password and press Ctrl+Alt+Del to reboot.