http://www.rabbit.org/care/babies.html
Hi,
You don’t say how old this baby is and whether it was weaned from its mother. How many weeks old is it? That makes a difference in what you will feed it.
I would suggest taking it to a vet immediately to have a diet plan recommended. The link above is an article from the House Rabbit Society website. Scroll down until you get to Orphaned Baby Bunnies. Read it from there (and read the rest later if you can.)
If you are talking about a 6-9 week old baby do not give it cat’s milk. Just give it a crock bowl of water, some alfalfa hay and a few gentle veggies, like parsley or green leaf lettuce. (Do not ever give Iceberg lettuce.)
Go up to the green banner on this page and mouse over the drop down menus. Read the sections on diet and care. There is a lot of info. on all the necessary topics for starting out with a bunny, especially a baby. I would also urge you to read the different sections on the HRS website. (www.rabbit.org) that I linked above.
Until we know more there’s not much else we can do, but a vet can help out immediately. Keep us posted.