What should I use to reattach loose tile?
To re-secure ceramic tiles it is best to consider what environment it will be subject to. If, for example, I am re-attaching a loose wall tile in a shower stall I wouldn't use a "mastic" type adhesive.  I've seen repairs fail when mastic gets wet.     For shower stalls with a solid backing go with any fortified thin set mix. Some people have claimed construction adhesive works, but as with the mastic the dampness may affect the bond. In a pinch I've had very good results with 100 % silicone caulking.     Which ever one you decide on the location of the tile should be  clean from the original grout and adhesive. Scrape down to the cement or backer board and wipe with a wet sponge (let area dry if you use silicone). Press tile with buttered adhesive into the cleaned location, use appropriate tile spacers if necessary, support with painters tape (for horizontal repairs). Finally, check tile by pressing  with a level so it sits at the proper   height.