Some of these may work:
Remove everything to do with the Nvidia proprietary drivers.
sudo nvidia-settings --uninstall
sudo apt-get remove --purge nvidia*
Start from scratch.
sudo apt-get remove --purge xserver-xorg-video-nouveau
Reinstall all the things!
sudo apt-get install nvidia-common
sudo apt-get install xserver-xorg-video-nouveau
Reconfigure the X server.
sudo dpkg-reconfigure xserver-xorg
Thanks