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You should be very worried. Any time that a cat won't eat for more than 48 hours, you are in serious trouble. Their liver can fail and then the cat can die. It is called hepatic lipidosis where the liver thinks it is being starved and starts "grabbing" fat cells...and actually ends up killing the liver.
I suggest that you find an after hours vet so that you can find out what's going on.
Kidney failure is also common in animals this age.
Unfortunately, there is no over the counter remedy for this kind of problem.
Let me know if you have more specific questions about this.