A cat who doesn't eat for any reason is in danger of getting hepatic lipidosis (fatty liver disease).
You did not give a lot of information about your cat, but, bad breath, weight loss and decreased appetite could be from kidney disease, or a problem with the GI (gastrointestinal system). It could also be something simple such as dental disease (too painful to eat) or an upper respiratory infection (a cat won't eat if he can't smell his food), Your vet will be able to sort it out with diagnostic's.
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