I was using gdm3 in Ubuntu 18.04, after upgrading it stopped working (Similar screenshot in gdm3 does not start in ubuntu 18.04) I had no login screen.
Using tty I tried (from: Can't start ubuntu-desktop after upgrade to 18.04):
sudo systemctl start graphical.target
This worked for a while, the graphical interface was started, so for testing purpose, I closed my session and I was able to enter again, but after a reboot the problem was back again, so from tty I had to change gdm to lightdm. Is it worth trying to get gdm working again?