My nieces bunny just abandoned her litter, I spoke with a wildlife refuge lady & she said to feed them 3x a day, through a dropper. Get probiotic gel & give it every other day. Get kmr kitten milk, mix 1 part kmr with 1 part heavy whipping cream & 2 parts warm water. Make sure the milk is room temperature or a lil warmer. Dont keep the mixed milk any longer than 24 hours. The bunnies need the extra fat from the whipping cream & she said she has not had good luck with any milk other than the KMR milk. Put the bunny in a small container with a heating pad to keep it warm. Make sure after feeding them to rub their belly gently with a cotton ball with warm water on it to encourage them to go to the bathroom. If you drip a lil of the milk on their mouth generally they will lick it and start looking for more.
Edit: Everyone is saying to contact a vet. Our first thought was take it to the vet too.. However, our vet simply referred us to the wildlife refuge. There isnt anything a vet can do other than bottle feed the baby & most arent going to do it, if you find one that will, you will be paying an arm & a leg for the care...