Wool Block
This is mainly seen in Fuzzy rabbits. It is a collection of fur in their stomach. Since rabbits can't vomit, they get hairballs sometimes. The best way to prevent this is to keep them well groomed, and increase fiber intake. When I get wool block I use a small dose of mineral oil to "grease things up" inside and allow them to defecate it out. I use a syringe to get them to swallow it.

Bad Feed
It could be bad feed. Check the pellets and make sure they smell fresh and have the greenish color to them. Are the other rabbits eating them? Diarrhea is another sign of the feed being bad. I had this happen to me once as well. I used the same method as above with the mineral oil, to wash out their system and I threw out the feed. Always check the pellets. For the best feed check out the help page I have on Feeding to get the right amount of nutrition for your rabbit.

If your rabbit isn't eating, and it's not wool block, it could be a sign of serious illness.