How to Recover your Ubuntu password

First of all, if you forget your admin Ubuntu password, you will need physical access to the PC in order to recover your password.

Usually Ubuntu install in the grub menu this option:

title        Ubuntu 8.10, kernel 2.6.27-7-generic (recovery mode)
uuid        393ac665-f5c2-488d-b601-b59ba1d5675b
kernel        /boot/vmlinuz-2.6.27-7-generic root=UUID=393ac665-f5c2-488d-b601-b59ba1d5675b ro  single
initrd        /boot/initrd.img-2.6.27-7-generic

That text is part of my /boot/grub/menu.lst of my Ubuntu Linux, so first go to the easiest way to recover the password in Ubuntu.

1. Reboot your system
2. When it is starting, press ESC to get the grub menu
3. Select the option that says (recovery mode)
4. In the next dialog select the root prompt, and get access to a console shell
5. type

passwd admin-username

Remember to change the bold text by your username in Ubuntu, the one that has admin rights.

6. reboot your system, and you are done!

Now Going to the not so easy way, in case you do not have the recovery mode option.

Follow the same procedure as above until step 2.

3. Press e to edit
4. Select the line that starts with kernel...
5. Press e again.
6. Go to the end of the line and add single
7. Press ENTER
8. Press b to boot that kernel, with the single option.
9. Change password, and reboot.

Hope it helps.

[Ref: http://www.go2linux.org/how-to-recover-ubuntu-password]