There are several questions and answers around this, and none seem to help.
I upgraded a laptop with an Nvidia NV31M GeForce FX Go5600 video adapter from Ubuntu 12.04 to 14.04. I have tried the nvidia-173 driver, the nvidia-common driver and the xserver-xorg-video-nouveau driver with the same results.
When I boot the machine, it comes up and displays my walpaper on the screen the menu bar at the top and a login box and Ubuntu 14.04 at the bottom left exactly as I expect. At this point, ethernet is working and I can ssh into the machine and everything is normal from the ssh terminal.
If I try to log in on the console, either as myself or 'guest', I still see my wallpaper and Ubuntu 14.04 as I expect for some seconds. Then, at the point where it would normally display a desktop, the screen goes black. I have a bright white cursor which I can move with the mouse or touchpad, but I see nothing else on the screen.
I can still communicate normally with the machine via ssh over the ethernet port, but I see nothing but the black screen and cursor on the laptop display.
There is an external VGA port on the machine, and I have connected a monitor to it with essentially the same results.
What more information can I provide to help figure this out?
Note 2015/06/05: Additional information. When I originally upgraded this machine from 11.10 to 12.04, I recall that there was an issue involving Nvidia and X11, and at the time I found a very helpful installation package that included an X11 downgrade of some kind that I think was required to make this work, although the symptom wasn't a black screen. I no longer recall what exactly I did, nor can I find anything either on the computer or online about this.