I don’t know what happened...

My shih tzu just passed away. A couple months ago he was having trouble walking and wouldn’t jump up on the couch or anything anymore. The vet said he may have hurt his leg and to just watch to see if he got better. A couple days later he was back to his normal self. Now fast forward to about two days ago. He was having trouble walking again and was also peeing in his room a lot even after he was taken out. Today he was so off. We let him and our other shih tzu out their room and into the living room and then got them ready for a bath. After he was washed he just fell over and took a breath. After that he stopped breathing completely. We have no idea what happened to him..... does anybody know what could have happened??