Is your dog eating? If so, then there has to be something a bit wrong, because if something is going in, something has to come out the other end! If you think your dog might have chewed something inedible (like a toy, furniture,stick, clothes, etc) there might be an intestinal blockage. Usually that would cause your dog pain, and vomiting most likely. So you would be able to tell your dog was sick.
If he/she hasn't eaten anything, then there will be no pooping.