At the login screen press CTRL+ALT+F1 and login as normal user, then execute:
No errors should happen (if errors happens write them here!) and your prompt will be with an "#" character at the end, you are root now: don't do stupid things and run
exit to return a normal user.
After this check let's reconfigure your login manager, execute:
sudo dpkg-reconfigure gdm and, eventually, select gdm as your default login manager.
sudo killall Xorg; after that you will be asked to login (if don't press CTRL+ALT+F7) without any error: try to login.
If it still don't work, press CTRL+ALT+F7, run
passwd and change the password; execute
sudo killall Xorg and try to login with the new password.
If it still doesn't work (damn! :-) ) return to the virtual terminal and execute
rm ~/.Xauthority, then
sudo killall Xorg and try to login.
The password asked with "sudo" is your user password, never forget your password.
If the login still doesn't work, it's a more complicate case, report here any error.