Do you have a save from before that happened?
The file isn't there. I gave up and just reinstalled everything, because that's how easily I give up on such things. (The new version did have the file, though.)I got it a couple of times as well. Go to the mod folder. Click on the navigation bar and add "/.git". (For me, the navigation is "C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization IV Beyond the Sword\Beyond the Sword\Mods\RFC Dawn of Civilization\.git") In that folder you will find the file index.lock. Simply delete that file.