Greetings margreg welcome to the Forum !
One of the most important things is making sure that the wall is dry behind the Tile.
If it's a very frequented area, you might tape a piece of Plastic over the open space, just at the top, letting the bottom and generous sides of the Plastic remain open, where the area can dry out. The intent of course, is to "umbrella" the space, shedding moisture until it can dry thoroughly.
I've used Silicone Adhesive several times on problems like this. Clean out the space behind the Tile a bit more than you need, scrape the excess Grout off of the back of the Tile, and make sure when you apply the Adhesive, that you don't use too much !
Should be back from the edge of the Tile about 1/4".
When you push the Tile back in place, make sure it's square, and centered, then tape it in place, and wait till it drys well, still leaving the umbrella over it.
Then you can use a blade to clean out the troff around the tile if necessary, and apply the Grout, raking a gentle depression in it, where it matches the other Grout.
This way, you have a real good hold on the Tile with the Adhesive, and nobody will know, but your DIY friends and you !
Have a good Day !
S.W.
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