If your shower is made of interwoven fiberglass as most are, you can simply purchase a fiberglass filler compound and apply it generously over the crack. Smooth over to remove excess compound and allow 24 hours to dry. Use a medium grit sand paper to smooth it, and then a fine grit sandpaper to bring it to a mirror finish.
First clean the shower. As disgusting as this already is, you don't want any mold to take root. Not only use a shower cleaner, clean the shower once more and bleach the hell out it. Dry the shower, Next you're going to fill it with caulk. Lay down a coat or two of polyurethane.
How big is the crack? If it is not a wide crack, you probably can manage to fix it with help of some hot/cold silicone. I always have a tube of silicone at home to fix that kind of stuff. Silicone should be sufficient, if the crack is wider, you might need tiles as well.