House Trained Dog Started Peeing Inside?

We have had a Chihuahua for 4 years and he is the most well behaved dog ever, caught on to potty training quickly and has never had an accident in all of the last 4 years. Recently he has been peeing in the house and just simply has not gone in the potty in days. Nothing that we know of has changed and it just happened out of the blue.

We don't know why or how to even retrain him, any suggestions on why this may be happening and what we can do it fix it? A dog that never peed anywhere but the potty for 4 years and is now having multiple accidents and just refusing the potty all together is baffling to us!

By KB