Barbi Brown's Bunnies website gets this question quite often so here are some possible answers:

The first step is to check the water supply.  We need to understand that water is the single most important part of a rabbit's diet.   It must be cool, clean and easily accessible.  Bottles will not work properly if hanging crooked or clogged by debris.  If the rabbit has a bottle, try giving it a crock instead.  Don't assume because you fill the bottle and it is empty when you return that the rabbit is drinking it.  It may be leaking.

If the water is working and you are certain he is drinking, then look for the following:

If there feed dust collected in the feeder?  If so, clean it out and refill.

Is the feed in the dish wet, damp or "puffy" looking?  Moisture collects in a pellet very quickly and becomes moldy very fast.   Mold is toxic to rabbits.  Many are smart enough not to eat food when it is moldy but not always.  Throw out any old feed and put a small MEASURED amount in it's place.

GI Stasis or wool block caused by ingesting hair may be the cause of loss of appetite.  If your rabbit is molting, be sure to brush him thoroughly and feed lots of hay.  Remove all pellets and just feed hay and water if the rabbit is molting.  The binders in the feed that hold the pellet together can make the blockage worse. 

Have you changed feed recently?  Rabbits are very picky eaters and all rabbit pellets may look the same but the formulas are quite different.  Rabbits can smell subtle changes and chemicals and preservatives that we may not notice.  Rabbit pellets get stale very quickly and lose flavor and nutrients.   Check the bag for a date.  If it is over 12 weeks old, throw it out. See if the rabbit will eat other things like whole wheat kernels, rolled oats or oatmeal.  If he eats these but not the pellets, then it is time to consider changing pellets.

If the rabbit is drinking but is still not eating in 48 hours, take him to a rabbit savvy vet as soon as possible.  Obstructions can be fatal.