I spent several hours, but finally got it to work.
Ubuntu 14.04 Desktop, Nvidia card: FX 3800
First, do Ctrl+Alt+F1 and login with your username and password
sudo stop lightdm
remove your nvidia drivers and use your nouveau driver
sudo apt-get remove --purge nvidia*
sudo apt-get purge nvidia*
sudo apt-get install xserver-xorg-video-nouveau
sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf_bkup
now install unity-tweak-tool since unity -reset is deprecated
sudo apt-get install unity-tweak-tool
sudo apt-get install -reinstall xserver-xorg-core libgl1-mesa-glx
nvidia drivers have some dependencies with ubuntu-desktop, I also did:
sudo apt-get install ubuntu-desktop
and reboot
sudo reboot now
now do again ctrl+alt+F1 and login
up to this point, some of the steps that are similar to other posts on this #issue ... but when I tried reset unity from shell:
unity-tweak-tool --reset-unity
I received this error:
error: Cannot autolaunch D-Bus without X11 $DISPLAY
which I could not get around it, so
I install gnome-desktop, which worked (could see the desktop icons)
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install gnome-shell
sudo apt-get install ubuntu-gnome-desktop
now reboot and login, you will see a small circle icon on the right top side #of the username box, click on it and a menu will allow you select gnome, #(ubuntu unity is defaulted)
finally, once in gnome, open a terminal ctrl+alt+T and do
sudo dpkg-reconfigure xserver-xorg
sudo reboot
login to gnome again and open a terminal
setsid unity
reset unity
unity --reset-icons
dconf reset -f /org/compiz
unity-tweak-tool --reset-unity
this last step above will overlay unity on top of gnome enviroment but voila, now reboot and login to ubuntu unity and it should work, this did the trick for me:). The gnome unity overlay will disapear next time you login to gnome too. Maybe some of the steps above are not necessary but they didn't hurt either. How I got into this mess to begin with? Trying to install cuda toolkit. Hope this helps!