Boot stuck - how to start GUI from the command line?

During boot, my system is stuck for some reason. I can get to the command line by clicking Ctrl+Alt+F3. After login, what can I do to start the official GUI of Ubuntu 20.04? I tried the following:

 gnome-session
 gdm3
 systemctl isolate graphical 

but none of these worked.

I also tried:

lightdm

and I did see a graphical login screen, but not the usual one; and I could not log in from that screen (I got "authentication error").

I am looking for the most standard and officially-supported solution - I don't need any fancy GUI managers - I just want to get back to work...

lightdm is not the standard Gnome window manager, it is gdm3, so there is some misconfiguration. Easiest way to try to remedy it is to, when logged in, run:

sudo apt-get install --reinstall ubuntu-desktop

and reboot.