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
I had a terrible experience trying to reinstall the default drivers in ubuntu.
After applying (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)
I ended up with a not functioning graphical subsystem which means I had to re-install ubuntu from scratch and then apply backups.
There is an easier way to revert
Nvidia provides an application "Nvidia X Server settings", if you were to look at the options, it provides a way to revert back to default intel drivers. You select the option, then reboot and voila you are back to default settings.
Don't use the command line would be my advice.