Tile floors are designed so that each of the tiles, and the grout between them, work together to seal out moisture from the tile base and the underlayment. One loose floor tile can become the first breach in a series of them. Unless you fix it right away, the loose tile will allow moisture under the adjacent tiles. If the loose tile isn’t cracked, you can reuse the same one; otherwise, you’ll have to match it at your local home improvement store.
Scrape out all the grout from the perimeter of the loose tile with your grout saw. Dig the grout out completely without scratching the tile surface. Use a vacuum hose to pull up the pulverized grout.
Set the edge of the putty knife into the open grout line alongside the tile, at a shallow angle. Tap the handle of the putty knife with a hammer, to wedge it under the edge of the tile.
Pull the knife out from the edge of the tile, move it over and tap it under again. Continue working around the edges of the tile with the knife, gradually getting it more and more loose, until it finally pops out.
Scrape the exposed wall with your razor scraper to remove any remaining mortar. Scrap the back of the tile as well.
Spread mortar on the back of the tile, using your putty knife. Cover the back completely, to a depth of about one-eighth inch. Press it in place on the wall, getting it evenly spaced with the surrounding tiles.
Allow the tile to set for eight hours. Spread grout around the edge of the tile with a grout trowel, pressing it into the space. Use a dampened sponge to wipe up the excess grout. Let it set for 48 hours before you walk on it.