I, I've installed Ubuntu 11.10 on my dual screen PC and everything worked flawlessly.
Then they asked me about proprietary drivers: ATI.
I said "ok install them".
Now it doesn't work on my dual screen anymore, I've got only one "duplicated" screen.
So, my question is: how to re-install the original video driver?
To properly revert the graphic drivers to the out of the box state do the following commands in a terminal.
Now reboot your system and everything will be fine.
You would simply uninstall the display driver you have installed. Open
jockey-gtk
aka "Additional drivers", click on the activated driver and click remove (lower right corner). This should revert the settings to your former ones.