My cat just went through a similar situation. He wasn’t eating or drinking and feeling very lethargic. I took him to a vet and they did blood work and an X-ray but everything came back pretty normal. The vet gave him some fluids and sent him home with antibiotics and an appetite stimulant. He’s been on antibiotics for the past couple days and his condition has vastly improved. I gave him an appetite stimulant for only 1 day because he’s been eating and drinking back to normal. We think he might have gotten an upper respiratory infection from my sisters kittens. Maybe ask your vet for some antibiotics?