Install Gnome Display Manager if it is not already installed
sudo apt install gdm3
Stop and disable the currently display manager, replace sddm with the current display manager. May be lightdm
sudo systemctl disable sddm
sudo systemctl stop sddm
Enable GDM
sudo dpkg-reconfigure gdm3
sudo systemctl enable gdm3
sudo systemctl start gdm3
Should be good to go.
This post is not meant to be a how to, but rather a compilation commands that could be helpful in trying to fix issues with the login screen after an upgrade.
These commands should be executed from a prompt, you can get to one using the ctrl+alt+f1 (you can replace f1 with any of the function keys from 1-7), or you can boot up from recovery, or you could mount the drive and chroot into it.
Reinstall Desktop and Login Manager
sudo apt-get install --reinstall lightdm ubuntu-desktop
If your using using Gnome Shell
sudo apt-get install --reinstall gdm3 gnome-shell
Reconfigure lightdm, set it as the default login manager
sudo dpkg-reconfigure lightdm
Replace GDM with LightDM
sudo apt-get remove gdm3 sudo apt-get install lightdm
Installing GDM (Gnome Login Manager)
sudo apt-get install gdm3
Remove Intel video and Plymouth
sudo apt-get remove xserver-xorg-video-intel plymouth
Reboot, get to a prompt and reinstall login manager, replace lightdm with whatever one your using
sudo apt-get install --reinstall lightdm
Reinstall Intel video and Plymouth
sudo apt-get install xserver-xorg-video-intel plymouth
Install Ubuntu Gnome Desktop
This seems to solve some of the issues with GDM3 not starting up.
sudo apt-get install ubuntu-gnome-desktop