1. Push and hold the"Shift" button when booting into to the OS to enter the GRUB menu.
2. On the "Ubuntu" item, press "e" button to go to editing interface.
3. Add " rw init=/bin/bash" behind the "linux ... ... ... ro quiet splash".
4. Press "Ctrl + x" to boot to command line.
5. Enter "passwd root" to reset the root user's password.