Likely was clipped by a car.
Any chance you can get some injectable steroids? Especially if there is any swelling, this can help them out a long way towards recovery. Personally, we've got a fair number of hit snakes back up to speed: a male Kisatchie that lost both hemipenes and part of his tail, another on that struck a car that passed by it and really tore up a jaw bone (small meals after that!), a Louisiana milk that had its head ran over (actually, I did this one....sigh), etc.
KJ