If you update your graphics driver you will get this message, cfs3config needs to be run again when you update drivers, and reconfigured - and as Olham says it's stored in Appdata not the one in the sim install directory.
See FAQ near the top section anyway - it's mentioned there.http://www.overflandersfields.com/FAQ.htm#Installation_and_settings
Dropping the xml file back in, in this case won't work as the top part contains info relating to the new video driver. But if you are comfortable editing files it's possible to retain your current settings from the major part of the ConfigOverrides.xml if you back it up first, before running cfs3config, as it's usually just the top part that is needed for the new driver. Others may be able to help with that it's not something we can "support" directly - and if it breaks we'll tell you to run CFS3config properly.
this bit at the top is what needs to change normally the last 4 lines there, but bad editing can break...
But best to just follow the FAQ if you are not comfortable doing those things.