HOW TO REATTACH A TILE THAT IS FALLING OFF?

our suggestion is to try to remove the grout between that tile and the other in order to be able to insert a nozzle under the tile (use a disposable medical syringe of the largest available size).

You will have to go to a hardware store and buy a (the smallest), universal adhesive that is mostly polyurethane based, choosing between the more liquid type and slower drying type. Attempt to fill the cavity formed between the base and the detached tile with the adhesive and the syringe.

If it is not possible to get under the tile because of a narrow gap (space between that tile and the other), the only solution would be to remove it.

At this point you could either try a very fine bladed knife or a saw blade, removing the grout between that tile and the other, doing this on all four sides of the tile until removal is possible - taking good care not to break it. Then spread the same adhesive as above on the screed and stick the the tile down again. Wait for at least a day and then refill the gap using grout than can be bought a in DIY shop (alternatively, use cement mortar).