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...