I have a five year old Shih Tzu that I believe is marking in the house. I just learned that yelling at him is not the correct thing to do. He is very intelligent. I don't know what to do with this dog. It used to be if we said "pee pee" to him he would turn around to go outside. But now he insists on going inside the house. He doesn't want to go outside anymore. I am at my wits end. Our wood floors are getting ruined. Help

Is he neutered? And has he been checked out medically to see if he may have a urinary tract infection or some other medical problem?

I agree with Lady Sam. The first step is to take him to the vet for a checkup, to rule out any medical issue that may be causing this.

If he's not neutered, this could also be what's causing the sudden lack of potty training.

Have there been any major changes in the home, a new pet, a new job where you're gone more often, a new baby, a new roommate, anything? It could be as simple as a new couch.