First, backup your xorg.conf
:-$ sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf_backuppersonal
Then, try booting into recovery mode and typing into the terminal:
Just go with the defaults if you don't know what you're doing. It'll require a little fixing later, so don't be freaked out if it looks like the drivers don't work when you get back in.
:-$ sudo dpkg-reconfigure xserver-xorg
If that doesn't work, you can try
From there, we'll see where else we can go.
:-$ sudo dpkg-reconfigure gdm