→ → GNOME GUI not starting
GNOME display manager (login screen) does not start, only black screen is showed
System successfully booted, but GNOME Display Manager (gdm—login screen) did not start, login prompt is displayed:
In some cases it might help to reconfigure the GNOME login manager. Login with your username/password at the prompt:
and run the following command:
sudo dpkg-reconfigure gdm3
Restart to see if it helped.
If the problem happened after a system upgrade, you can try Fixing broken system upgrade.