Looking in on your rabbit one morning, you notice that he doesn't need new food because he didn't eat the food you gave him the night before. While this can be worrisome, don't panic. Observe your rabbit for a day or so and see if he exhibits any other symptoms of illness. If he stops drinking water, too, you may have a problem that needs immediate attention.
Check your rabbit's feed and water supply. If it got wet or is otherwise spoiled, your rabbit may be refusing to eat it, but is still hungry. If the water is dirty or unpalatable, this can also cause him to not drink.
Clean your rabbit's water bottle completely. Take apart the lid and wash the rubber ring separately. Make sure it is thoroughly rinsed and does not have an odor when replacing.
Offer something different for your rabbit to eat. Your rabbit could simply be bored with the same old pellets or need different nutrients. Give him hay, lettuce, broccoli, spinach, a piece of banana, a sprig of fresh parsley or rolled oats to sample.
Take away his food. It's quite possible that your rabbit has been overfed and needs to "appreciate" meal time a little more. Take away the food in the morning and don't give it back until later in the evening or even the following morning. If your rabbit still shows no interest in eating, there may be a medical issue.
Eliminate any stresses in your rabbit's world, including excessive heat, a new rabbit roommate, or noise and clamor during meal time. Try placing his cage or hutch in a quiet area and see if he just needed a break.
Look in your rabbit's mouth and see if there is a dental issue that you can find. An abscessed or broken tooth could prevent your rabbit from eating.
Take your rabbit to the vet if he won't eat or drink for several days and you still can't find the cause. Your vet can help you get nutrients and fluids into your rabbit and help you find out what the problem is.