A: Unfortunately, it’s not as easy as ‘sticking’ these tiles back to the wall. Water has gotten behind the tile and ruined the drywall behind them. The paper face on the drywall, which is effectively what the wall tiles were adhered to, has disintegrated. What’s left of your drywall has gotten wet and dried out and is now brittle. If you re-adhere the tiles, and grout them, they likely won’t stay put for long.
It’s time to remove the tile and cut out as much of the drywall as was damaged and replace it. If that’s a sizable portion of your shower surround, it’s time to consider a new shower surround. Matching tile sizes and colors is difficult, and mismatched sizes or colors look like an obvious repair.