To recover it you need to follow these steps:
1. Boot your computer.
2. When you get the grub menu, select option with recover as shown in figure.
|Ubuntu 12.04 : Grub Menu|
|Ubuntu 12.04 : Recovery Menu|
4. A shell prompt will be displayed at the bottom of the screen with root as username. To change the password of particular user say naveen
5. The above command will ask you to enter your new password and retype password. Type your new password.
6. You might get "Authentication Manipulation error". If you get this error then you need to type :
mount -rw -o remount /
And again follow step 4 and 5.