How do I reset a lost Administrative(root) password?

To do this you need to boot into recovery mode.
Boot up the machine, and after the BIOS screen, hold down the left Shift key. You will then be prompted by a menu that looks something like this:

Hit the down arrow until you select the 2nd entry from the top (the one with the recovery mode in the description) and then hit Enter.

Using the arrow keys scroll down to either root or netroot (doesn't matter in this case) and then hit Enter.
You should now see a root prompt, something like this:

root@ubuntu:~#

Now we can set the user's password with the passwd command. (In this example I will usesurabhi as the example, you need to substitute whatever the user's username is):

passwd surabhi
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
root@ubuntu:~#

Type in what you want the new password to be at the prompt. After it's successful reboot the machine and the user will be able to log in with their new password.
It worked for me.

Reference http://askubuntu.com/questions/24006/how-do-i-reset-a-lost-administrativeroot-password