I had this happen recently on 2 of my VMs after running the updates.
I believe that there's nothing wrong with the root user or password but that gdm was reset to disallow root login.
This appears to be the case as I was just able to fix it on one of them by logging in as another (non-root) user, opening a terminal, and running:
Code:
su -c 'gedit /etc/pam.d/gdm-password'
You'll be prompted for the root password (and you'll notice that it works). Next find this line and comment it out:
Code:
auth required pam_succeed_if.so user != root quiet_success
Save and close gedit. Then either reboot the machine or restart gdm (service gdm restart). You should be able to login as the root user now without issue.
If you don't have another user setup, you should be able to use the grub menu to edit the boot parameters and specify runlevel 3.
From here, you should be able to accomplish the same thing logging in directly as root and using vi and nano to edit the gdm-password file.