It's a serious matter if a rabbit stops eating. They need constant roughage going through them, or they'll die. If your rabbit loses its appetite and its poop pellets get small and dry or stop coming, it is a sign of wool block. Some readers here call it "constipation," but that's not what it really is.
[01/05/2009: Joyce from Arcadia, CA, USA writes: "I WILL TRY THE PUMPKIN FOR MY SAD CONSTIPATED BUNNY. I HOPE IT WORKS."]
It's actually much like hairballs in cats, only since bunnies can't vomit, the fur can get stuck inside their gut and actually kill them. You have to get the gastric track moving again as soon as you see this problem start. If your bunny stops pooping, or if her pellets are starting to look small and dry, that's a sign she's blocking up. Lack of appetite is another symptom.
My vet, who is a rabbit specialist, has me keep a product called "Critical Care" on hand for wool block emergencies. This is even better than the pumpkin treatment. I get my Critical Care from my vet but you can probably get it online or at your pet food store, especially if you ask for it. You mix a little of this stuff with water, suck it up into a big syringe (one about the size of your middle finger, being sure to remove the needle and toss it in the trash before you work with the bunny!). Then you put the plastic tip of the syringe into the side of the bunny's mouth and VERY SLOWLY squeeze out a little at a time. They will like this and swallow it, if you don't disperse it too fast. Wait a few seconds between each dispersal. Only give about a couple teaspoons for each dose, then wait about three hours and do it again.
Pumpkin can be fed the same way if your bunny has wool block. Make sure you use pure 100% canned pumpkin NOT canned pumpkin PIE filling, which has spices in it and could hurt your bun! I've found the Critical Care quickly eliminates wool block (you give it 4 to 6 times a day until they start eating and pooping normally again). I used to do the pumpkin treatment until I found the Critical Care, and pumpkin worked pretty well, but failed to work with one bunny who I almost lost to wool block on account of "pumpkin failure." I brought her to the vet in time (you've only got a couple of days to save them if they stop eating), who gave her Critical Care, and the bun was fine in just a couple of hours. Believe me, this stuff is AMAZING.
The vet said my buns get wool block because I wasn't feeding them exactly right. In my case, it was too many vegetables. Now that I'm feeding the right diet and giving them more exercise, they've been doing fine. (Exercise and plenty of water are important for maintaining intestinal motility - the constant movement through of food. So is brushing them when they're shedding.)
Right diet for a rabbit means unlimited quantities of timothy hay (or orchard grass) always available to the rabbit (you can get this at a pet store but ordering it online is much cheaper. In some parts of the country, feed stores sell timothy.) You also have to feed about a half cup of fresh vegies a day for a medium-sized rabbit (3/4 to 1 cup a day for a giant breed). Certain vegies, though, will kill bunnies if fed over time, so choose from the "safe vegie list": green pepper, collard greens, swiss chard, parsley (a little), cucumber (a little), cilantro, endive, mustard greens, lettuce (NOT iceberg), carrots (only a small slice a few times a week), broccoli (only a tiny flowerette a couple times a week), certain weeds including dandelions, chickweed and plantain (if not subject to exhaust fumes from cars). Visit www.rabbit.org for the full list of diet do's and don'ts. You can supplement this diet with a bit of daily timothy pellets, available from Oxbow (NOT alfalfa pellets - these are only for babies, and fed long enough they can kill an adult).
Make sure your bun has unlimited access to water - a crock is ideal for most bunnies, as they can then drink all they want. I put 3 drops of vinegar in my rabbits' water, and now they love drinking.
In case anyone reading this has a pet bunny living in a cage, please know that buns can easily be litterbox-trained and make wonderful house pets. I have house bunnies living happily alongside my two house cats. You have to introduce them to each other gradually, of course. All sorts of tips on how to make your rabbit a successful house bunny are available on www.rabbit.org, the website of the nonprofit organization called House Rabbit Society. If it weren't for them, my buns would have been dead by now. I made so many mistakes at first, having no one to show me. I have six precious buns, and I've had 5 of them for six years. Most buns only live a year as pets, studies show, because owners make mistakes with their diet or other errors with these delicate creatures. It's very easy to keep bunnies if you're taught how to do it, and very easy to lose them if you're not. Check out that website and also the chat room "Etherbun." Between those two places, you'll find all the info you need. There are some snotty people on Etherbun, though. Don't let them hurt your feelings if one of them gets nasty. Just visit there for your bunnies, learn what you can for their sake, and don't let the bullies get to you.