Is this a one-piece plastic shower stall unit? If so, they frequently are designed to only be attached in a few points to the wall so as to permit things to flex. I would try to verify the source of the water damage; if you have a vent or a skylight over the shower, check that it's sealed properly to the roof and pull the interior trim if possible to check for signs of water intrusion there.
To glue things in place, if that's how they were originally mounted, I'd look for a construction adhesive that's compatible with the shower stall material. PL Premium is one that I've used with good results for various tasks (though not specifically for shower installation).
Some pictures of what you're up against may help get more specifically useful responses.