I know that you're not going to like my answer because I can't give you good advice on how to treat this at home.
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 see the vet first thing for a full exam and possibly bloodwork to find out what's going on.
If you can get her to eat something today, it's not as much of an emergency. You can try a little chopped boiled chicken or jarred baby food meat. You can also use plain pedialyte to rehydrate her.
But don't hesitate to give your vet a call if she doesn't start eating and improving soon.
At this age, renal failure and liver disease are very common. But they are treatable with drugs.
I also worry that the cat has suffered from a blood clot, based on what you have described.
I truly recommend urgent vet care for you, if you want to save him.
Let me know if you still have questions.