u/Individual-Fox5795

I have a large shower in my basement. I am guessing that it was installed in 1995 when the basement was finished.
The shower is on concrete slab and against a concrete wall on two of the three sides.

One 2x2 tile popped off a year ago and we haven’t used the shower since.
How do I approach fixing it? I have done a lot of tileing but not in showers like this. Is there cement board under tiles when already on cement? There is no obvious leaks and never was. Just a tile that had grout wear down and the tile popped off on the floor.

What would my next steps look like to get the shower functioning again?