Nvidia driver installed successfully, but not activated

I was also facing the same problem. Now that you asked it, I wanted to fix in my computer as well. So here is how you do it.

EDIT: sudo apt-get install nvidia-current

sudo nvidia-xconfig. This will just create the file /etc/X11/xorg.conf. Next do

sudo software-properties-gtk

You should see something like this

Select Additional Drivers tab and select the latest NVIDIA driver as I have selected here.

Now reboot. This reboot might take a long time. No need to worry. If it takes more than 10 minutes with the black screen, even after rebooting, restart with Ctrl+Alt+Del or even hard reset (keep pressing the power button as long as needed). It should work properly now.