Reset the forget root password

1.Reboot your computer, and then as soon as you see the GRUB loading screen,
  make sure to hit the ESC key so that you can get to the menu

2.Select the second line (Rocovery Mode) and press "e"

3.Move the cursor to kernel line, press the "End" to the end of the line

4.Remove the "ro recovery nomodeset" and replace it with "rw init=/bin/bash", then Press F10

5.Reset the root password

6.login system with the new root password