You can recover/reset/change your root password on Ubuntu if you have physical access to the machine following these simple steps.

1. Reboot Ubuntu.
2. At the GRUB menu press ‘e’ (with quotes), which will let you edit GRUB.
3. Edit the line with your boot command.
4. Add this command to the very end of the line with the boot command:
   rw init=/bin/bash
5. Press enter and boot your system, the command, “rw init=/bin/bash” will make your Ubuntu boot with passwordless root shell.
6. Once you’re logged on just change your password with the passwd <username> command. If it is root just type: passswd root.
7. Reboot your system.