Make sure the baby has a fur bed, even if you have to pluck some from mama. Unfortunately, that may stress her, but it sounds like she already has had some stresses.. the chances of saving this one aren't great.
But get some fur in there if the mother hasn't taken care of it, and then leave them alone and as quiet as possible. Not knowing the circumstances... did you know she was bred? Did she have a nestbox? Was there a disturbance where she lives? (rabbits will abort if seriously frightened,.. if a strange dog gets into the rabbitry, you may come out to a scene of carnage- or only a whole bunch of open does who ate the dead offspring before you got there)
I don't want to insult you, (so if you've been breeding for years but just haven't had an orphan before, ignore it! LOL!) but if you are thinking of breeding rabbits and haven't done so, you want a good basic book. One which is rather outdated in some ways, but still was an excellent primer (although written by a rather autocratic purebred breeder of show rabbits!). It's called "Raising rabbits the Modern Way", by Bob Bennet. It won't help you grow them on a homestead after TSHTF (he's fond of wire cages and pellets, and no other possible options) but his info on rabbit reproduction is excellent.
Good luck
Summerthyme