A lot of people has trouble booting in to Ubuntu after installation of Nvidia Driver.
Finally I had found a solution to this problem.
On the boot menu Click
Ubuntu Recovery Mode
You will see a Blue Screen . On the options scroll down and click on failsafex
You will now see a small dialog box. Click on OK Button.
Press OK two or three times more.
You will now boot into your Ubuntu OS.
Connect to the internet.
Open Hardware Drivers and install(activate) a recommended Nvidia Driver.
Reboot and boot into Normal Ubuntu and your Ubuntu will boot just fine. If this doesn’t work ask me by commenting on this post or email me.