They die, either starvation, organ failure or dehydration or a combination all of which can happen very, very quickly without proper medical care. Cats systems don't work like ours and organ failure can begin if they don't eat for even as little as 24 hours. Any time a cat doesn't want to eat or drink it warrants a phone call to the veterinarian at the minimum and usually a check up to try and diagnose and offer appropriate help asap.