Before panicking and trying to get a hold of a vet, check your rabbit's water. Is it empty? Is the water dirty or does is taste funny? Is the nozzle broken or frozen, therefore keeping the rabbit from getting any water?
This happened to me before. Gracie wasn't eating and I was terrified something might be wrong. But no, she was just low on water.
Rabbits won't eat if they don't have accessible water. Therefore, water is the #1 MOST IMPORTANT thing you can give your rabbit. Check the water everyday. Depending on the size of the rabbit and the size of the bottle, you may not need to refill the water every day. But you should check to see that water is coming out easily and that it is not dirty.