First of all make sure to get access to a root shell;
Pressing ESC as the computer BIOS screen closes, some say that holding a SHIFT key also works, to get access to the grub boot menus. In there you can navigate to an option that reads "recovery console" - and press RETURN to run it.
(at the recovery console, select to run
fsck and then activate networking, before dropping to root shell - press enter one extra time to actually get the shell prompt! - this is the easy way, you don't have to remember any commands to get through)
Alternatively you might have luck and get a text mode login prompt if you press CTRL+ALT+F1 during the text mode boot (press esc to reveal it)... before the login gui appears. You will probably have to be quick to succeed with this though.
After login from text mode prompt; type these commands and keep an eye open for error messages. If something fails here, I suggest using a text mode browser (links, lynx, ...) to google the error(s).
Here is what I did to get back to a GPU-driven GUI:
When checking with
dpkg --list | grep nvidia it did show:
367.57-0ubuntu0.16.04.1 amd64 Transitional package for nvidia-367 ii nvidia-367
375.66-0ubuntu0.16.04.1 amd64 Transitional package for nvidia-375 ii nvidia-375
384.111-0ubuntu0.16.04.1 amd64 Transitional package for nvidia-384 ii nvidia-384
384.111-0ubuntu0.16.04.1 amd64 NVIDIA binary driver - version 384.111 rc nvidia-opencl-icd-361
367.57-0ubuntu0.16.04.1 amd64 Transitional package for nvidia-opencl-icd-367 rc nvidia-opencl-icd-367
375.39-0ubuntu0.16.04.1 amd64 Transitional package for nvidia-opencl-icd-375 rc nvidia-opencl-icd-375
384.90-0ubuntu0.16.04.1 amd64 Transitional package for nvidia-opencl-icd-384 ii nvidia-opencl-icd-384
384.111-0ubuntu0.16.04.1 amd64 NVIDIA OpenCL ICD ii nvidia-prime
0.8.2 amd64 Tools to enable NVIDIA's Prime ii nvidia-settings
361.42-0ubuntu1 amd64 Tool for configuring the NVIDIA graphics driver
Which shows a hive of different nvidia drivers all installed at the same time... I went on with uninstalling all of them, and then attempt to reinstall the previous version:
sudo apt purge nvidia*
sudo apt autoremove
sudo apt autoclean
sudo apt install nvidia-375 # makes 384.111 emerge *sigh*
... but as you see in the comment; that still brought in the "latest and greatest" version.
Instead of giving up at this point, I gave it a try...
... and the login prompt came up!
Now, leaving it like this might have disable auto-updates, (as the nvidia-375 package was specified explicitly). That remains to be seen though.