My cat is 18 years old. When I went outside to the backyard (where she prefers to be)to check on her I noticed that her eye was closed shut and the same side of her face is swollen. She won't eat or drink anything. What should I do?
You should take her to the vet. She likely has some type of wound (insect sting, infected laceration) and a vet will be able to tell what type of wound it is and how to treat it.
Further, if she hasn't been eating or drinking for longer than a day, she may be suffering from some of the complications of anorexia, such as dehydration or even hepatic lipidosis. These complications are serious and life-threatening, especially in an older cat.