[ossiblue]

Those are edge tiles and they're held on slightly differently. Look at the remaining tile and you'll see the nail head through the side of the tile, nailed into the side of the rafter. You'll also notice the hole in the tile further toward the front. Those are the areas where the tiles are secured. if the fallen tile doesn't have a hole, it's probably because it was cut too short in order to fit the distance between the overlap and the flashing. It looks like the roofer may have secured it with the glob of mastic you see in the top of the overlapped tile. If the piece didn't break, you can drill a small hole in the side an nail it up. If it is broken, cut a new tile to the proper length, drill a hole, and nail it up.