Ctrl+Alt+F3 and login into the shell.
ls -lah. If in the output the line
-rw------- 1 root root 53 Nov 29 10:19 .Xauthority
then you need to do
chown username:username .Xauthority and try logging in.
ls -ld /tmp. Check for the first 10 letters in the left: they should read exactly so:
drwxrwxrwt 15 root root 4096 Nov 30 04:17 /tmp
Else, you need to do
sudo chmod a+wt /tmp and check again.
If not both, I’d recommend you either
or uninstall, reinstall it.
Alt+-> until you reach the login screen again, and restart.