If you can't switch to tty by ctrl + alt + F1 combination you have to launch rescue mode. You can do it by holding shift after post test (before ubuntu start launching). Now select you want to have network connection and go to root command. You have to remount your drive from read only to full access:

mount -o remount,rw /

Now purge your current nvidia drivers installation by:

apt-get purge nvidia*

and restore older driver:

apt-get install nvidia-current

Now you can try to reboot.

(and in farther way install e.g nvidia 331 drivers)