Hi,
I have alerted the other forum leaders who have more experience with abandoned, unweaned babies and wild rabbits then I do. In the meantime, keep her complete safe and away from your cat (obvious, I know, but a cage if you have one would be a good idea, or in a box in a room with the door closed.) Get a nice soft blanket or towel for a bed, and I would have a dish of water there, too, along with a bit of hay in a corner for litter. You will probably have to syringe feed the baby water and some food soon, but I need to wait for someone else to let you know iexactly what is best at this age. If you have a syringe, try to get a bit of lukewarm water into her – just a few drops to start. If you don’t, I’d run out to an open drugstore to get one.
Find where a local rescue is, preferably a wild life center, in your area. Your best help and advice will come from the local animal rescue in your city or town. Sometimes a lactating mother with a small litter has been rescued and can take on an orphan.