If your cat won't eat, take special notice if he is vomiting, having diarrhea, acting lethargic or having any other signs of illness. If he is, call your veterinarian to have him evaluated.
Methods to encourage a cat to eat may include trying different brands of canned food, tuna, baby food (avoid flavors with onion), or heating food up in the microwave so the food aroma is released. If your cat continues not to eat, make sure you call your veterinarian.
Not eating is one of the first symptoms of disease or illness in cats. For more information on why cats may not want to eat, please read Anorexia in Cats.