Since
rabbits grow so fast, a difference of a week in age makes a huge difference in viability for young kits. If the doe doesn't make it and the kit is only 2 weeks old then you may be able to save it by hand feeding it 2-3 times a day. Rabbits
milk is very nutrient dense and high in calories so the kit will need something comparable in order to meet it's needs. honestly I don't know what the best choice would be but I've read that some people use the supplements made for kittens whose mother has abandoned them. Maybe call the
local animal
shelter to find out if they have a good idea. This can be fed by using a syringe from the pet store or the ones that come with kid's over the counter liquid medications. Kit usually nurse for about 5 minutes 3 times a day. They
should have a nice full round belly after being fed. It'll need to be kept warm in a nest of fur. I assume that must still exist. With no litter mates you may want to keep an eye on the temperature and feel the kit to make sure it's nice and toasty warm to the touch.
If the kit is 3 weeks old and is hopping around the cage with it's eyes open then you'll have much better luck in saving it. At about this time they start eating pellets and
hay and will also drink
water from the dispenser. They ideally would stay on nursing until they are about 5 weeks old then they pretty much will be fine on pellets. I'd still try to give it some kitten milk or whatever is recommended by a shelter or vet.
I try to breed at least 2 does at the same time so that if either one has any trouble I can foster some kits to the other mother. I recently had a doe give birth to 14 kits. Sadly she couldn't
feed them all and 3 have died. Another one of my does was bred at the same time and should have been able to foster the extra kits. She didn't take during the first breeding and I ended up getting her bred a week later. Sometimes the timing just doesn't work out. In this particular case my trouble was doubled. The second doe gave birth on a night when the temperature was -20F. She's not a good nest builder, and in this case it cost her the whole litter. I found them all frozen on top of a small pile of fur.
Losses and setbacks are tough. I hope you are able to save your rabbits. Best wishes