http://www.bictor.com/2015/10/07/enabling-root-user-in-ubuntu-14-04-3/

By default root user account is disabled for login in Ubuntu. Steps mentioned below will allow you to enable the root user and login as root on the OS.Login to your account and open Terminalsudo passwd rootType in the new password for UNIXsudo gedit /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.confAt the end of the file append greeter-show-manual–login = trueThat’s all, now reboot the system and you can login to root account using manual login.