The first thing to do with pets that are not eating properly is to take them to a vet, unless you know the cause of their refusal to eat.
Pets, as they age, can become finicky.
After a lifetime of eating the same food it is possible that they may want a little variety in the brand of food you purchase.
You do not have to change the brand, unless it is causing health problems, but you can switch around the flavours. For example, if you feed salmon and ocean fish, you might change to venison and lamb. For wet food, you might consider adding as many flavours as is sold in that particular brand. You may get chunks, pate, and shredded styles as a way to get more flavours for your pet. It depends on what you are feeding. You always want to stay with the same food brand, unless otherwise directed.
If changing the flavours does not help, then you definitely need to visit a vet to figure out if there is an underlying issue.
The first thing that could stop your pet from eating is a tooth problem. Tooth decay hurts. Any cavity your pet has could make chewing very painful. Gingivitis can also be causing inflammation that is uncomfortable when your pet tries to eat.
What is the cause of the gum disease?
Plaque can be one cause. This is typical if you do not provide dental care for your dog or cat. However, there is also a disease that comes from fleas called Bartonella. Bartonella causes gingivitis, fever, conjunctivitis, and can lead to death in your animal. Pets rescued from shelters are more apt to get Bartonella because of the many animals at the shelter and the possibility of fleas. Bartonella is a disease that can take years to start showing any symptoms.
Intestinal issues, whether it is a stomach tumour or lower in the GI tract, such as kidney crystals, can cause a change in how your pet eats. Worms, as mentioned above, are not generally a cause of undereating, but overeating. Only blood work, stool samples, urine samples, and physical exams will be able to reveal why your pet is not eating if they stop.
If you decide to change your pet's food before taking your pet to a vet, look to see if they are eating again beyond two weeks. Some cats and dogs will start to eat again because they are excited over the new flavours, but will go back to not eating due to discomfort after a week or two.
Pets that are not eating may also have a behavioural issue. They may be worried, depressed, anxious, or suffering from any other behavioural problem that lessens their desire to eat.
Finding the cause is the only way you can correct the issue.