What Would Cause A Cat To Stop Eating?

It is always a cause for concern when pets start acting off especially when they stop eating.  Although he may not have vomited he has most likely stopped eating because he is feeling nauseous.  There are many, many things that can cause mild gastrointestinal upset leading to nausea.  Stress, dietary indiscretion, pain/discomfort, constipation, infection, and more can cause nausea. 
If he is still not eating tomorrow morning I would have him examined by a veterinarian.  Cats can develop hepatic lipidosis (fatty liver disease) secondary to not eating.  This can have a fairly fast onset-sometimes within a few days.  This is more common in overweight cats that stop eating.
In the meantime you can give him some Pepcid to settle his stomach and encourage his appetite.  The dose for most cats is 1/4 of a 10 milligram Pepcid tablet--this can be given twice daily.
Monitor for other signs of discomfort, water intake, drooling, vomiting, and make sure he is using the litterbox for both urination and defecation--especially if he is not producing urine this is an emergency.

I agree with the doc....but I notice that my boyfriends cats get a little suggish in the summer (they are also indoor cats ) and eat much less than in the winter months....if he is really off his normal day....right visit your vet...by the way... His cats are in the southern usa,  where it's even still warm in the winter but really hot in the summer.

How old is the kitty? Are you sure he didn't eat something around your house? I would just suggest that you get him seen, as there could be something like internal bleeding, and that could be part of it. Hope this helps, good luck.

Thanks.That sounds reasonable. I know sometimes they just feel off,like we do,just as you said. I just get worried when Sam doesn't want food,he's such a chow hound!

He might not be in the mood to eat, we all feel like that every now and then. Just keep an eye on him and make sure he is drinking freash water and if he is not eating after a couple of days seek vet advice. Hope this helps

She is sick and doesn't want to eat. My suggestion is to take her to the vet as early as possible and give her proper medication.