To piggyback on what
@GoDucks said, cats not eating can quickly become a medical emergency.
My cat Luna was on a very long stretch of antibiotics to fight an upper respiratory infection that she just couldn't seem to shake. The antibiotics screwed up her gut flora and she lost her appetite, which resulted in a $900 vet bill (emergency + feeding tube insertion), and we had to feed her a slurry of food through a feeding tube inserted in her neck over the next week. She bounced back pretty quickly with a different antibiotic (doxycycline vs. long-term amoxicillin) and thrice-daily tube feedings with a high-calorie "critical care" food.
She had gone from normal to early fatty liver in about 48-60 hours.
I cannot stress to cat owners enough how important it is to watch their food intake and to monitor for changes in appetite and litterbox habits. Not eating and trouble urinating (urinary blockage/bladder stones) can become life-threatening emergencies incredibly quickly.
If your cat is put on antibiotics for a respiratory infection, please ask your vet about pro/pre-biotics, and monitor your cat's food intake closely for the duration of the antibiotic treatment.