After trying to switch from Intel driver to the Nvidia graphics driver on my notebook computer, using Software Updates option from the gnome-control-center, it now boots only to command prompt with no attempt to display a graphical login screen. I tried all the "terminals", using Ctrl+Alt+F1, F2, and so on, but none has the GUI login prompt.
I tried some answers in AskUbuntu, like reinstalling ubuntu-desktop, using the tasksel command to (re)install desktop, removing and reinstalling NVIDIA drivers, trying to switch back to Intel driver, using systemctl set-default graphical.target, but none of them helped.
I have done similar graphics driver switches (Intel ⟷ Nvidia) in the previous versions of Ubuntu for various reasons and never had this problem. My system was upgraded to 22.04 from 21.10 in the end of April. I am using this very system since 18.10 and upgraded every six months with no Ubuntu re-installation so far. The system is fully updated.
The startx command opens a tiny white terminal at the top left corner of the screen. I can run the command gnome-shell to bring up GNOME, but it is very slow and some GUI tools like gnome-terminal or gnome-control-center just crash.
I also noticed that the directory /etc/X11 and its sub-directories of are all empty now. It may be a problem with Xorg.
The problem was fixed after purging and reinstalling
gdm3:After the above, I noticed that a file named
/etc/X11/default-display-managerappeared and it has the following contents: