Tile? Preform pan? What's wrong?
we need more info if you want help.
This article might help you! https://homesteady.com/how-10073440-redo-shower-floor.html
If cleaning it doesn't work well enough you'll need to replace it. Is the floor done in ceramic tile? or is it a fiberglass/plastic preformed pan? Home Depot has a ton of options for you to look at. Good Luck.