I have Ubuntu 16.04. Some packages were updated automatically. After restart graphics resolution changed. When I try to login, system return me back to logon screen without any warnings. When password is deliberately incorrect, it also returns to this screen, but with error that password is incorrect.
I tried to get into advanced load option, and choose 4.-38 version instead of current 4.-42, and everything loaded correctly.
How to make it works normally, at least - log in.