Comment 32 for bug 1779827

Same issue on the workstation, few hours spent to fix it by reinstalling gnome, gdm3, apparmor, loading previous kernel, switching to lightdm. Nothing helps, complete reinstall. Very frustrating, because need to return to configuration/installing apps for next the few weeks.
With Ubuntu you should get used to always have the drive with recent version near you in case of something go wrong. Because from my experience it is faster to reinstall than fix such issues.