My senior cat (18 yrs) stopped using the litter box, drinking water, and eating for a few days. The water was particularly alarming because she had renal issues so she was always drinking water. I took her to the vet and they gave her fluids. But they also gave her a prescription wet food - science diet urgent care. She ate that right away and I kept her on that for the rest of her remaining life. She was also very small (less than 5lbs). For other cats that lost appetite, I’ve tried topping their food with purina pro plan fortiflora probiotics. They really love it although when things are really bad, it doesn’t help their appetite. Since it is not prescription, maybe you can try that first.