bearded dragons do show black bars or spots on their bellies naturally, usually these indicate the dragon is stressed or just not happy. If your dragon hasn't eaten and is just brumating (though it is rather late for dragons to be brumating now, most are already awake, though not all dragons brumate at the same time), he shouldn't be showing signs of stress.
As another poster suggested, if you are at all concerned with teh health of your dragon, taking him to a vet is always a good thing to do. Bearded dragons that are lathargic, don't eat, are showing signs of weight loss or dehydration, should see a vet.
It is also very hard to give advice when so little information is provided. Not eating and not being active could mean brumation but without a picture of the animal to see what it looks like, or knowing the basking temps, and other husbandry information, not to mention the age of the dragon, makes it difficult to give more meaningful advice.
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PHLdyPayne