Hello it sounds to me like Nvidia-config has caused troubles.
For certain systems after installing the Nvidia drivers and running Nvidia-config it is possible (and more likely for dual screens) for your xorg.conf file to be generated incorrectly.
For certain (OLD) systems:
/etc/X11/xorg.conf specifies how the desktop should be displayed on your screen(s) if this is invalid you will not be able to boot into Ubuntu.
For this scenario I suggest you drop to main terminal
(Ctrl+Alt+F1) (you can do this even if your desktop is not showing) and navigate to
/etc/X11/ where you should have
xorg.backup.conf or some such (a backup file) you should be able to restore one of the backups (which is generated by nvidia-config). This will revert back to the old 1 screen configuration and allow you to access your desktop and fix the real problem by:
For most systems, I would recommend a reinstall of the drivers and after the re-installation attempt to auto-config again.
For installing the proprietary drivers there are 2 "good" approaches:
- You can attempt to install via the ubuntu-x-swat repository. This is the simplest approach. Instructions can be found here.
- You can attempt to install the latest drivers by downloading them from the nvidia website. Then dropping to main terminal
(Ctrl+Alt+F1), stopping the display service
"sudo servive gdm stop" and then installing the drivers
"sudo ./NVIDIA-versionxxx.sh". Towards the end of the installation you will be asked if it should auto configure say yes and hopefully this should work and you will have both screens ready to go.
Option (1) is easier to do and will automatically update the drivers as you update the system (essentially less maintenance).
Option (2) will require you to reinstall the drivers every time the kernel is updated more maintenance but you will always have the latest proprietary drivers.
Hope that helps.
If you decide to attempt Option(2) after Option(1) remember to remove the old drivers by
"sudo apt-get remove nvidia-current"