If your cat is acting sick and not wanting to eat, the safest thing is to take your cat to your veterinarian and allow them to evaluate your pet for an underlying cause. It is important not to overlook this important symptom of a problem.
There are cats with diagnosed problems that just don't feel well and want to eat. For these pets, there are several things you can try.
If your cat does not have interest in his everyday food, you can try offering something special. Sick pets may reach the point where some form of nutrition (within reason) is better than no nutrition.
Often offering a cat a small amount of a new food will stimulate their appetite and encourage them to eat. Consider your pet's illness before selecting the foods. For example, if your pet has been nauseous, offer bland foods. Avoid rich or fatty food and table scraps if possible. For diet recommendations for your cats particular problem, check with your veterinarian.
To encourage your cat to eat, begin by offering healthier choices, such as a quality, canned or semi-moist pet food, boiled chicken, tuna, etc. Meat-flavored baby food is a tasty, bland food that is good to entice a pet's appetite. Do NOT feed only baby food as a primary nutrient source, as it does not contain enough calories to sustain your pet. Make sure you avoid foods containing onion in cats because onions can be toxic.
If healthier food choices are not working, you can try some pet "junk" food. Lower-quality canned food (which is usually more palatable), sandwich meat, etc. are some possible choices.