Reasons why a cat suddenly stops eating the cat food it has been eating:
1. It's possible that the cat food hasn't been changed in a long time and has gone bad. Cats have a very sensitive sense of smell, and they won't eat if they smell spoiled cat food.
2. The cat may have gastrointestinal inflammation or be unwell. Observe for symptoms such as vomiting and diarrhea, and take it to a veterinary hospital as soon as possible if you find any.
3. It's possible that the cat is a picky eater, or that the owner has given it too many treats as rewards, and it's tired of the same flavor of cat food and wants to try something different. 
What should I do if my cat suddenly stops eating the cat food it's been eating?
1. You can try fasting your cat first and observe whether it is not eating because it is not hungry or has eaten too many snacks. If so, you need to give up the snacks.
2. You can try to entice the cat with palatable food and observe whether it is willing to eat.
3. Increase your cat's exercise to expend its energy.
4. Switch to a cat food that your cat likes and that is palatable. However, be sure to switch foods gradually and add probiotics when feeding the food.
5. If the loss of appetite is due to illness, it is necessary to take the child to the hospital to check for the specific cause and treat accordingly.
