Comment 21 for bug 1767468

I had these exact same symptoms today with my nVidia GTX 960 after doing a platform upgrade from 17.10 to 18.04.

Issue was actually dead simple to fix, the upgrade to 18.04 removed some PPAs (including the graphics drivers), so although the nVidia driver package was present on the system it wasn't getting updates and the system appears to have selected a software only driver to use instead. All I did was uninstall nvidia-390 using apt, re-added the ppa, apt-get updated, then reinstalled nvidia-390 again using apt. Fixed it fine for me.

###
sudo apt-get autoremove nvidia-390
sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt-get update
sudo apt-get install nvidia-390
###

hope this is useful :)