I followed the steps in this answer entering these commands as I had a similar problem...
sudo apt-get install --reinstall ubuntu-desktop
sudo apt-get install unity
sudo apt-get purge nvidia* bumblebee*
sudo apt-get install nvidia-prime
sudo shutdown -r now
...but, after rebooting, I lose video signal during the boot sequence before the log in screen.
How can I get the video signal back? I have Ubuntu 14.04 and nvidia graphics card.