first make sure that the back of the ile is clean - secound make sure that the wall where you are reatching the tile is in good shape (still solid). There are a few product that you can use to put the tile back on. The least expensive is "POOL PUTTY" this is an epoxy product that will hardden above or below water. when first mixed it is verry simular to clay it will the get rock hard. the other product is an adhesive sealant such as Aqua bond, this is more expensive and requiers that you buy a gun and mixing tips to use this product.