After upgrading to 14.04 few people face issue of really low graphics.
Need to do few steps :
On Black Screen,
Press ALT + CTRL + F1
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
After restart, it will be fine 🙂