During the pc boot up process, when grub boot loader appears press “e” to edit,
find the line which begin with linux and on the end add init=/bin/bash

example: linux /boot/vmliznu-3.2.0-4-686-pae root=UUID=8cbe7996-107b-4d22-9281-f0962e4917dd ro initrd=/install/initrd.gz quiet init=/bin/bash

press ctrl+x to boot, this will drop you on the command prompt

mount -o remount,rw /

passwd

reboot