I noticed a tile in my shower was loose, so I figured I would pull it off and reattach it. When I pulled it off I found water had gotten behind the tile and deteriorated the backer board behind it. So now there is nothing to attach the tile back too.
Do I have to replace the entire wall of backer board? Then re-tile? Or is there a way to replace just the lower portion for the tiles to attach too?
I'm sure there is no leak as the board is intact and fine above.
Something caused the substrate material to fail. Make sure you identify the source of the water intrusion before sealing it back up. As far as how high to go, go high enough until you have solid material behind your tile.
Some advice you receive on the internet can be worth exactly what you paid for it - be careful.