Cats do not have whims like people. Whenever they stop eating they are either ill or in pain. If she has no other symptoms like fever, diarrhea, constipation, infection, etc then here is something going on that is difficult to ascertain - but it exists nevertheless. Sometimes a cat can have an abscessed tooth or root that is in the gum.They can ingest a piece of plastic that in not perceivable in an xray. They can be mildly dehydrated or have a metabolic or organic imbalance that is too slight to read on a test. She may also feel nauseated.
With a cat her age,, I am suspecting a bad tooth, however i would hesitate to subject her to any anesthesia for a denta procedure unless her bloodwork supports her ability to tolerate it.
She may just be dehydrated or hypoglycemic.
If your cat is not diabetic begin by giving her two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When she feels hydrated she will lick up the gruel on her own.
Boil chicken or fish as the scent of food cooking awakens the appetite sensor in the brain.
Please get started ASAP and let me know how things are going. I will stand by to hear from you and help further.