Tile on Master shower wall falling out

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My master shower is tile on 2 1/2 walls. One of walls at the bottom , the tiles caved in kinda and I was able to pull them off with my hand. Behind the tile it appears that the mudd stuff is crubbling and you can actually see the wood stud. It does not appear to be wet but there again I am no expect. Where do I start in trying to repair this myself, if that is even a good idea?