Just now, I used the Ubuntu software updater to install a bunch of updates, and now I cannot log into my account. Unfortunately, I can't remember which programs were updated and I can't get into my account to check. When I enter my password, the login screen takes me to a giant command terminal, but the error messages scroll by much too quickly to be able to read them. (The Guest account can be accessed fine, however.)
My question is, how can I revert all these updates without being able to get into an administrator account? Or is there another way to get back in without knowing any very specific details about what the problem is? My computer is a System 76 laptop running Ubuntu 16.04 LTS.
EDIT: I managed to use the command line and the startx command to log in, but I still can't do much except open command terminals...and those terminals can't find simple commands like cd and it doesn't recognize the Xsession-errors file.
EDIT2: I did also manually install a couple programs since the previous restart: android studio, Segger, Kicad, Slack, and Discord. So those may also be responsible.
It turned out to be a problem with LightDM. For anyone coming across this problem, I ran the following commands and was able to log in properly afterwards:
Source: Posted by OP in the question itself.