I'm unable to enter tty to use sudo service lightdm start, as the normal boot will not even get me able to press ctrl+alt+f1 to f6. How do I fix this?
I had a very similar problem where I got a blank screen when booting from an Ubuntu 16.04 Live USB on a GTX 1080, and found my answer in another question: Ubuntu 16.04 + Nvidia Driver = Blank screen
Super-short version: Add
nomodeset to the boot options and then reinstall the NVIDIA drivers.
Most likely, CUDA installed NVIDIA drivers that don't work with your GPU. Assuming you get to the GRUB menu on boot, press
e to edit the boot command for Ubuntu, then add
nomodeset to the options (it usually has
quiet splash by default, so make it
nomodeset quiet splash, for example). This should fix the graphics, but if not try adding
nvidia.modeset=0 to the options as well.
Once you get into a terminal, you can install the correct drivers:
sudo add-apt-repository ppa:graphics-drivers/ppa sudo apt-get update
sudo apt-get install nvidia-367, assuming you want the latest drivers (as of this writing), or
apt-cache search nvidia to see what's available. Once the new drivers are installed, you should be able to reboot without the nomodeset option and everything should work.
A note when installing CUDA: If you're ever reinstalling CUDA, I recommend using the the runfile method and answering NO when it prompts to install drivers, as I very much suspect the drivers are what caused the problem.