If its been more than 12 hours since your rabbit last ate, you need to take it to the vet immediately. There may be more than one cause for it to stop eating, but the most serious problem is that after a rabbit stops eating for so long, it goes into stasis, or ileus, and its digestive system slows down and then eventually stops. It takes a lot of time, medicine and money to get it going again if it gets to that point. Save yourself the hassle and heartache and get your rabbit checked out immediately.Answer Is it really hot out? The bunny wont eat a whole lot when it is hot. If this is the case blow a fan by it(NOT directly on it) or you can freeze water in an empty pop bottle and give it to him to lean against, just make sure you peal the label or your bunny will eat it.
Does he seem to show an signs of illness. Maybe you should try to switch the type of food you give it. If this bunny came from someone else, try and see what type of food it ate there and get that kind.