If you have extras of the same tile, carefully chip out the cracked one, removing grout too. Work gradually so you do not damage the surrounding rules.
Use tile adhesive or silicone to stick a new rule in place and regrout with matching color.
If that is too much, just run a bead of silicon down the crack and let cure- this will keep things water proof till you can manage a full repair.
How to Repair Holes or Cracks in Ceramic Tile
Patching unwanted holes in glazed tile like this can be a tricky proposition. Here's a solution that may not always be perfect, but will make a big improvement. Polyester resin or auto body filler, often used to repair car bodies. Mix up a small batch by adding some hardener and stirring it thoroughly. Using a putty knife, press the resin into the hole slightly overfilling it. Then just as the material begins to harden while it's somewhat rubbery, trim off the excess with a razor blade.
Next use 600 grit sandpaper to sand the filler smooth. Wetting the paper with water makes it work best. Now it's time to try and match the color of the tile. A porcelain touch up kit which is available at most home improvement centers comes with several different colors of paint. You can also use artists acrylic paints available from Michaels, Hobby Lobby, art supply stores, and online.
The trick here is to mix these different colors together until we come up with one that matches the tile. Apply the paint with a small brush. You can test it on a nearby tile. When you get it the way you want it, then just paint over the patch and let that dry. Give it one final sanding with that very fine sandpaper and you'll have, well, maybe not a perfect match, but something that's a heck of a lot better than what you started with.
It depends on the type of tile materials. What type of tiles are they? Ceramic, vinyl, terracotta clay, natural marble/stone/quartz, or something else. Take a photo of your flooring (and if you have a spare tile) to a home improvement store like Lowes, Home Depot, Mennards, etc. and get expert advice from them concerning the best way to repair your floor.
You are better off redoing the entire area, than trying to do a quick flash patch. Its not worth your time and money.
The bigger issue is, if water is getting behind it. If you think so, then get the complete section replaced. I dont think with tile there are any short cuts.