I think this might be helpful... I installed Nvidia driver and restarted my computer. I got black screen in loop. The solution is to use other graphic card to boot. Cd or usb is not required for this. My solution is:
As computer starts keep on pressing left Shift key.
In order to edit grub2 press e.
Then go to line starting with linux and enter i915.modeset=0 before quite slash.
Since my choice was to use the Intel graphic card. I inserted that line. If it was amd/ati, radeon.modeset=0 could work. In some computer simply inserting nomodeset may work.
Press Ctrl+x to reboot.
Then wait for a while... After screen freezes press Ctrl+Alt+F1
It asks for username and password. Enter them.
Then terminal appears. Uninstall NVdia drivers using
the command:
sudo apt-get purge nvidia*
Then type reboot... then your Ubuntu will boot...
This solution is just to enter the OS. Now follow proper method to install NVidia driver for better graphics.