Install Nvidia Drivers from .run File Provided by Geforce.com

I was having the same problem, and I fixed it by running the following commands in the terminal:

sudo apt-get purge nvidia* ## remove the previously installed proprietary driver
sudo reboot
sudo add-apt-repository ppa:xorg-edgers/ppa
sudo apt-get update
sudo apt-get install nvidia-340

That worked for me. Let me know how it goes for you.

Same problem, here, last week my laptop LG with Intel Sandy Bridge and Nvidia GForce start freezing in the way you describe. In the past it could freeze from time to time. I move around with the laptop, so at home and work I use a dual screen but in the train (from home to work and back). I don't have the external display. ;-) Now every time I connect/disconnect the external display unity freezes.

My solution Ctrl+Alt+F1:

setsid unity

This allows Unity to be responsive again, but I miss the menu bar. Then I open a terminal in Unity and put again

setsid unity

and it works till the next time I unplug/plug the dual screen.

I'm also thinking to move away from unity, but I need to find a good desktop that looks nicely with dual screens of very different resolutions.