Press Ctrl+Alt+F3 and login into the shell.

Now run 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.

Else, do ls -ld /tmp. Check for the first 10 letters in the left: they should read exactly so: drwxrwxrwt.

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

dpkg-reconfigure lightdm

or uninstall, reinstall it.
Now press Alt+-> until you reach the login screen again, and restart.