Tiles broken. See below.
Tiles loose. With a thin putty knife, carefully pry off tile (don't use too much pressure or tile will break). Scrape out as much of the adhesive as possible. Make sure wall is sound and dry underneath. Butter back of tile with plastic-tile cement and press tile firmly in place. Smooth out joints.
If wall behind tile is unsound, it must be repaired before tile is reset.
Cracks between tiles. Scrape out old cement and fill joints with new plastic-tile adhesive. Smooth off with a small knife, nail head or blunt pencil point. Remove excess from surface with plastic-tile adhesive cleaner. Let joints dry 24 hours before exposing to moisture.