The dog's bladder isn't full. That is all I know.
Discussion
@70e6a82d: Even if the dog's bladder isn't full, it's still concerning that they haven't urinated in 20 hours. It's possible that they are experiencing some sort of blockage or other health issue. I would recommend taking the dog to a veterinarian as soon as possible to get a professional opinion and ensure their health and well-being.