https://askubuntu.com/questions/1037922/ubuntu-18-04-hangs-on-booting-with-message-started-user-manager-for-uid-120-on
Enter Ubuntu by recovery mode, from the main menu choose the first option,
the one about the restart, give always ok and you should arrive to the desktop;
once in, update and upghrade everithing and after that open a terminal and type
"sudo nano /etc/gdm3/custom.conf",
once the file is opened, in the [daemon] section
uncoment "WaylandEnable=false" ,
save and restart, problem fixed ;-) .
で解決