Boot into recovery mode
Boot the computer from an Ubuntu live USB, and after the BIOS screen, hold down the left Shift key. You will then be prompted by a menu that looks something like this:
The timing when to hit the left Shift key can be tricky, so sometimes if you miss it you need to try it again.
Hit the down arrow until you select the 2nd entry from the top (the one with the recovery mode in the description) and then hit Enter.
Now you should see this menu:
Using the arrow keys scroll down to root (the second entry from the bottom in the Recovery Menu) and then hit Enter.
You should now see a root prompt and you can run commands from it.
lightdm is called lightdm because it is a lightweight application compared to GDM which is heavy and takes longer to load. If you still have lightdm installed you can switch back to using lightdm as the default login display manager with the command:
sudo dpkg-reconfigure gdm. This will open up a new window allowing you to select either gdm or lightdm as the default login display manager.
Reboot the computer with the command
After that the keyboard shortcutCtrl+Alt+F3 may start to work properly, and allow you to start Ubuntu from the console.
Start Ubuntu from the console
Open a text-only virtual console by using the keyboard shortcut Ctrl+Alt+F3.
login: prompt type your username and press Enter.
Password: prompt type your user password and press Enter.
Now you are logged in to a text-only console, and you can run terminal commands to reinstall the NVIDIA driver from the console. To reboot the system after you reinstall the NVIDIA driver run the command:
sudo reboot. Press the keyboard shortcut Ctrl+Alt+F7 to exit the virtual console. In Ubuntu 17.10 and later press the keyboard shortcut Ctrl+Alt+F2 to exit from the virtual console.