I was stuck in a similar situation. I removed all Nvidia drivers and associated programs and then manually re-installed the drivers from terminal mode. I believe, in my case, that an unknown Nvidia program conflicted with the Nvidia upgrade (3.19). With the screen black you will need to use terminal mode CTRL+ALT+FI to conduct all your work.
In my case, the initial re-boot "failed" after installing the updated driver (3.19), but I shut-down and rebooted and everything worked. What I have heard (unverified) is that the Nvidia graphic card is not immediately recognized by the computer when booting, hence initial failure.
I did a quick check and ran across this website: Nvidia Driver 331.20 released, Install it in Ubuntu & Linux Mint. It may prove helpful.
This may prove helpful too: Install NVIDIA GeForce driver in Ubuntu 13.10 / 13.04 / 12.10 / 12.04 using PPA
How to change proprietary video driver using the command line?
Also read the following: Reset Unity and Gnome to default values [duplicate] and How To Reset Unity And Compiz In Ubuntu 13.10 Or 13.04. I found them useful too.