It's looking at /etc/X11/xorg.conf, so to revert you need to see if there is an xorg.conf.old or similar. You could just swap them and try it.
I belive it searches for "devices" in the xorg.conf file like this:
It finds the "ServerLayout" section and finds the Screen - which may be "screen1".
It searches for a "Screen" section with an Identifier that matches, i.e. "screen1".
In that "Screen" section, it finds the Device - which may be "device1".
It searches for the "Device" section with an Identifier that matches, i.e. "device1".
I think that is what it can't find, the "Device" section with an identifier that matches.
But if it found it, it would read the Driver line to see "nvidia" and use that driver.
You may be able to edit xorg.conf to fix it, or google or post here for very specific answers.
BTW, try swapping "nv" and "nvidia" on the Device line. That is, if one is there, try the other.