Though grout can be repaired and you can re-caulk joints, if the shower pan is leaking, you'll need to have it repaired by a tile installer or roofing contractor. Contact your homeowner's insurance company; repairing the damage may be covered. To check the shower pan for leaks:

1) Remove the shower drain screen.

2) Plug the drain with an expandable plug, purchased at a hardware store or home improvement center.

3) Fill the shower flow with water to about 1 inch from the top of the step and mark the water line. Wait a few minutes. If the water level goes down and wetness shows up beneath the shower, the pan is leaking.

Get Pre-Screened Bathtub or Shower Installation or Replacement Help