 People joke about cats and their finicky eating habits, but it's actually a serious issue if your cat won't eat. When animals don't eat enough, they must rely on their fat reserves for energy. Before stored fat can be used for fuel, it must be processed by the liver. This step requires adequate supplies of protein. Loss of appetite is one of the key indicators that something is wrong. So be sure to pay attention if your cat suddenly stops eating. A number of different conditions may be responsible, including infections, kidney failure, pancreatitis, intestinal problems, and cancer. But it isn't always serious, something as simple as a toothache can make your cat stop eating.