I don't know why you are having this problem, but one theory is that the drivers did not install correctly. Here is the method I used to install the latest nvidia driver using both the CLI, and Envy.
1. sudo apt-get remove nvidia*
2. sudo apt-get install envyng-gtk
2. enable the "proposed" repositories in sources.list
3. sudo apt-get update
4. sudo envyng -g
5. Select Nvidia on the left side, now click on Install the Nvidia driver (Manual Selection of the Driver), select 173, and finally click Apply.
6. Once the installation is completed, reboot.