An animal may experience appetite loss for many reasons, some minor, some major.
If your pet is hot, tired from exertion, ate too much the day before, had a switch in pet food, or was vaccinated, you probably don’t have to worry. Vaccinations 
cause nausea and loss of appetite. And dogs and cats in the wild had a feast or famine type of lifestyle. It’s also in their genes to not eat when they’re feeling sick. The natural fast acts as a cleanse.
If you think your pet’s appetite loss may be caused by some other reason, or if the loss of appetite lasts more than a couple of days, see your veterinarian. There may be something seriously wrong internally. Don’t force your dog or cat to eat.
Herbal and Naturopathic Help
If your pet ate something bad and an upset stomach is causing the appetite loss, giving probiotics with some fresh, raw chicken may help their digestive system to right itself. As soon as the animal starts to feel better, the appetite should return.
Make sure plenty of fresh, filtered water is easily accessible. Use glass, ceramic, or stainless steel dishes with no toxic paints or shellacs and make certain the bowls are kept clean.