It is clear that whatever backer you have behind your tile has failed. Is the home relatiely new? Some contractors cut corners by using greenboard behind tile. After 3-5 years the tiles fall off because the greenboard turns to mush and disintegrates. If the home is quite old then there is probably a mortar bed behind the tile (like my house). Mortar beds fail after 30 years or so.
In either case, the entire tub surround needs to be removed and rebuilt. Be sure to use a concrete-based backerboard as the base for your tile. Hardi-backer would be the best choice, as it is totally waterproof. Durock or Wonderboard will be "ok" second choices, though they aren't fully waterproof. But at least water won't damage them. Be sure to CAULK all joints in the corners and where the tile meets the edge of the tub. Never grout these areas, as the grout will crack due to movement and allow water inside your walls. That is one reason why your tiles may be failing now.