Oftentimes when people find a wild rabbit or a wild rabbit's nest, they think that the bunnies are abandoned and need help. It is rarely the case that a nest has been abandoned. Wild rabbits do not sit on their nests and only nurse their babies for a few minutes each day. It is uncommon to see a wild rabbit mother near her nest.
So, what do you do if you find a wild rabbit's nest?
It's best to leave the nest as undisturbed as possible. Keep small children and pets away from it to help protect the babies and to not scare away the mother.
If the nest has been disturbed and the babies seem to be cared for (stomachs not sunken in, moderate activity level), rebuild the nest as best as you can. Try to return it to its original state and leave the babies where they are. Leave the area so as not to scare away the mother.
* It's typical to not see the mother hanging around the nest. Wild rabbits do not sit on their nests and only nurse their babies for a few minutes each day at dawn and dusk.
If it is imperative to move the nest, choose a new, safe place in the same area, and attempt to replicate the nest to the best of your ability (i.e., create it at the same depth in the ground, move the fur from the original nest, etc.). The mother will most likely be able to find the new nest and will resume caring for her young.
Attempting to nurse the rabbits to health yourself can be very dangerous for the babies. Success rates for orphaned rabbits with inexperienced rehabilitators are typically quite low (below 10% survival rate past one week).
If you find a young wild rabbit that has fur, eyes open, and is out of a nest, it's very likely that it is just exploring and has not been abandoned. Baby cottontail rabbits have a full coat of fur within a week of being born, open their eyes at 10 days, and are fully weaned at three or four weeks. Just because you see a rabbit on its own, it does not mean that it needs help. Again, leave the area so as not to deter the mother from returning.
Bottom line is that as long as the rabbit doesn't require emergency veterinary care, it's best to leave wild rabbits alone and, if applicable, return any moved or disturbed babies back to the place you found them. If the rabbit does require emergency care, please contact your local vet immediately.
For more information (and more thorough explanations), visit the House Rabbit Society's page on orphaned/found wild rabbits: http://www.rabbit.org/faq/sections/orphan.html