Forgot Ubuntu root password - reset administrative password

You have to boot in to Recovery Mode. Reboot the machine and press down shift key until you see the GRUB menu showing different boot options. Choose recovery  mode.

Now you will see recovery menu.

Choose option

root     Drop to root shell prompt

You will see a root prompt.

The file system in read-only at this point. Make it writeable by command:

mount -rw -o remount /

Now you can use the passwd command to change any user's password.

For root just type passwd

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

For any other user use passwd <username>

root@ubuntu:~# passwd user1
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
root@ubuntu:~#

As long as you have physical access to the machine, you can use this method to recover password.