If your cat has stopped eating and appears to be losing weight and becoming listless, contact your vet as there could be an underlying medical problem. In renal failure and diseases causing fever, the 'appetite center' in the brain is affected causing the cat to lose interest in food. If the cat has a respiratory illness he may not be able to smell his food which will make him lose interest in it.
Sometimes cats get a toothache, just as we do. And nobody enjoys eating when we have a toothache.