Gosh, sorry to hear about this. Something is clearly wrong. How old is your dog? The best thing would be to try to carry him to the car and take him to the vet. All kinds of things can cause a dog to not want to move. What about a tic bite that has lead to sudden paralysis? He could have a disc issue in his back making it difficult to stand or move. Something could be going on in his brain which is more common in older dogs like a small bleed. You mentioned a firm belly. Have you changed dog food? Has he been into anything eating wise that could be causing distress? I had a friend whose dog ate a toy and then it was stuck in their intestines. All of this leads me to think your dog needs immediate medical attention and I would not delay. Take your dog in and let me know how he is!