On Ubuntu 15.10, after installing the nvidia proprietary driver, it didn't quite work as expected...going down to older version made it work, but graphics were slow. Run the below to reset back to the original graphics setup.
sudo apt-get purge nvidia-*
sudo apt-get install --reinstall xserver-xorg-video-intel libgl1-mesa-glx libgl1-mesa-dri xserver-xorg-core
sudo dpkg-reconfigure xserver-xorg
# May not have any alternatives configured (ok to skip)
sudo update-alternatives --remove gl_conf /usr/lib/nvidia-current/ld.so.conf