Im taking an Animal Care programin college right now we just had a guest speaker from a wildlife rehab centre come in a do a great presentation the other day and from what she said....
PLEASE PLEASE PLEASE put that bunny back where you found it!!!!! It may seem like the bunny has no mother but mother rabbits leave their young alone in the nests for days at a time, and only come back for a few minutes to feed them every couple days or so. Baby bunnies have no scent and the mother rabbits scent is very strong, so the mother knows not to go near to the babies very much since predators will then be able to smell them and the babies cant protect themselves. So please!!! It may look like the bun has no mother but it does!
You need to golook for the nest, which is usually just long grass and very well hidden and hard to tell it actually is a nest. Then put the bunny back with the others and to make sure the mother is coming back, take 2 pieces of string and lay them in an X shape over top of the nest. Then keep checking for the X to be messed up. The mother will come back to the nest in a few days and feed the babies,but then cover the nest back up and you will not be able to tell that shes been there unless you use the string.
If you can not find a nest, i would make a makeshift one for the baby and leave it there with some food. Use lots of long grass and plants and make it well hidden. Make sure to cover the bunny well so it cant really escape or be seen by predators. Use the string method as well.
If all else fails, or if the baby seems really really sick or unjured, take it to a wildlife rehab centre.
You say a cat got the mother? Did she die? You should check for a nest to see if there are any more babies if she did die and even if she did, put the baby back in the nest for a few days, with some food though, and use the string method. Sometimes other mother rabbits will foster the bunnies. But if no one has returned to the nest, then trytake the babies to the wildlife rehab centre. If you cant do that. Try to keep the baby in a box with some grass or hay with some water in a shallow dishand LIMIT your contact with it. Make sure its not noisy where its staying. In a while you should be able to let it back into the wild. It should be old enough to befine on its own.