This from a newbie to newbies.
After a hard reboot and Ubuntu (16.04) loading up, I suddenly lost access to the GPU. The drivers and CUDA were installed but not being utilized (when access ubuntu via the shell/terminal only). I could see from a PCI listing that the GPU was connected. Also I experienced the login “doom loop” for unity which apparently is not uncommon for NVIDIA drivers on Ubuntu.
I thought I was going to have to purge the drivers and CUDA and start again – the usual suggestion.
Can I suggest that if you are experiencing this scenario:
The Xfce4 desktop environment WILL STILL WORK and you will be able to login (if you don’t have Xfce4 you should be able to install it from the terminal.
Login to Xfce4.
Access “Additional Drivers” from the Xfce4 drop-down menu.
If NVIDIA drivers are indeed installed, you should see them as an option. In my case, Ubuntu had loaded up the Xorg drivers, not NVIDIA drivers. Select the Nvidia drivers. You’ll need to do a reboot.