How to Reset "root" Password Using Single User Mode in Ubuntu

While your system booting hold the Shift Key. You will get the GRUB screen like below

Select the first Grub entry and press e to edit the the Grub entry that splash screen like below

Find and change the line 

linux /boot/vmlinuz-3.2.0-23-generic-pae root=UUID=xxxxxxxx-xxxx-xxxx-xxxxxxxxxxxx ro  quiet splash to linux /boot/vmlinuz-3.2.0-23-generic-pae root=UUID=xxxxxxxx-xxxx-xxxx-xxxxxxxxxxxx rw init=/bin/bash

After changed above line Press crtl+x or F10 to boot. Now you logged in without password

Now you can able to reset the root password using following command

Enjoy....