Cat Health Problems can be tricky, but If your cat is an older cat and is not eating, try not resorting to euthanasia too hastily. It could possibly be the presence of a Cat Behavior Problem
A lady names Sophie posted the following dilemma on Yahoo Answers:
"My cat is 13 years old, and for about 2 weeks she has hardly eaten anything.
She has lost 2kg and is quite bony. Before she was a normal size, if slighly overweight, but always came in for regular meals and ate when she pleased.
She hardly eats anything and she has been to the vets three times, but they could not find anything wrong. We are considering putting her down, but I will not stop til I know why she hardly eats anymore. She eats about once a day if that, and she is eating a little tuna, and a few crunchy snacks, because that is all she will eat. Please help, I do not want my cat to starve to death, or have to be put down."
Cats sometimes tend to change their habits when they get older. However, since this was a sudden behavioral change, it might be worth looking at more closely sometimes with the help of a second opinion from another veterinarian.
Also, occasionally a vet might need to run all her blood work to see if a problem exists. Certain cats eat less when they get older and she may have lost her sense of smell a bit. Offering her strong smelling foods may help encourage her. It could be as simple as a sore tooth or an infection in the mouth. Cooked tuna, chicken breast or rice from the store are easier to chew. Often, trying something new other than her usual food also may help. If she eats dry food, moist or wet food could be attempted.
There also could be the presence of worms or an intestinal virus. Bringing the cat outdoors in a grassy field might also help the problem, since cats like to cleanse themselves from hairballs and other toxins by eating grass and plants. Cats cure themselves from these problems quite frequently. If the cat is not very old (less than 16 or so) you should keep on looking for answers until you find out what you can do to help her get better.