Assuming you can access the terminal, have you considered downloading and switching to another desktop environment? Recently I had a very similar problem with compiz/unity and the only solution was switching to GNOME 3. If you don't really care that much about the GUI, maybe that's the way to go.
So you can access your account and you can log into other accounts without any problem. I'm just guessing but maybe it's worth a try.
sudo apt-get install ubuntu-gnome-desktopBy the way, It can be a great moment to getting to know GNOME because the next LTS Ubuntu release will have it as default, from what I've heard.
