If your rabbit stops eating or drinking, it needs medical help immediately. If your regular "rabbit-savvy" vet isn't open, you should bring the rabbit to the nearest 24/7 vet. If you have experience in first aid, and you have the necessary supplies, you may be able to take care of the rabbit yourself until your regular vet opens in the morning. Either way, the rabbit needs fluids/hydration, force-feeding, pain medication, and gut motility medication - unless there's an actual gut blockage, in which case the rabbit might need surgery. Never give medication to a rabbit unless you're directed to by a vet, because you could end up making the rabbit even sicker. Many medications safe for humans (including babies), cats, and dogs are not safe for rabbits at all. Many medications that are safe for rabbits under some conditions, are not safe under other conditions, and only vets and very experienced rabbit people can know which is which. See the related questions below for more help and for useful links.
Your rabbit may be ill, and if it's seriously ill it may be giving up. When a rabbit gives up it will stop eating and drinking until it dies. So if I were you, take it to a vet now!
If your rabbit hasn't eaten in over 12 hours, take it to the vet immediately. There may be some underlying cause, but your rabbit is probably going into stasis, or ileus, where its digestive systems first slows down and then stops. It's hard to get it started again, and the sooner you start, the easier it will be. Make no mistake, if you leave this untreated, your rabbit will die.
It is important to take it to the vet as soon as possible. if it is possible you should try and give the rabbit water to drink to stop it getting dehydrated. when a rabbit stops eating/drinking it can be caused by spurs (spikes) growing on the molar teeth. A vet can burr these down which should sort the problem out.
12 hours is too long to wait. Your rabbit is getting more and more dehydrated. Try giving some baby pedialyte in a plastic eye dropper (glass ones can be bitten and break.) to your rabbit. If you can get some pedialyte in him you can stave off dehydration. Sometimes the pedialyte will stimulate appetite. Put some FRESH aromatic timothy hay in the cage to see if you can tempt the rabbit to eat a little bit, if he does, dry bread, carrot coins, cheerios and an apple slice can be tried also (not all at the same time). If you have mint in your garden or are growing carrots a sprig of mint and or a sprig of carrot top are good for your rabbit and mint will settle a tummy ache. If all of this is unsuccessful take your bunny to the vet, but don't wait 12 hours AND be prepared for a good sized bill.