Behavioural changes in Cat

Recently my cat's behavior has been changing and there is no infectious sign but mood and his behavior has much changed.What may be the reasons and how to solves this problems?

There might not be any visible sign that your cat is sick, they are REALLY good at hiding it.  How has your cat's mood and behavior changed and how long has it been going on?  Did anything change in your living arrangements around the time that you noticed his different behavior? A change in food? Cats are creatures of habit and like their routines; if his was upset in some way he might be stressing about it. Is he eating and drinking like normal? Literbox habits seem normal?