Recently, I tried the method in Can I access Ubuntu from Windows remotely to connect from a Windows 10 laptop to an xfce session on a workstation running Xubuntu.
After logging off that machine, when I tried to log back onto my own physical machine (in person), I was locked out of the GUI session ! Strangely, the guest xsession account was working. I could log on to tty1, and set up another temporary admin account from the command line.
I was able to login to tty7 (graphical user session) using this newly created account. From there, I was able to go to user accounts settings, and verify that my original user account still exists with all admin privileges.
I solved the immediate problem by using the method in Login screen won't accept my password :
sudo rm .Xauthority sudo reboot
But this leaves me with the question of what is going on in the first place. Can anybody shed light on what just happened ?