Depending on where the break is, almost all stock cracks are repairable to good as new.
Repairing a cracked stock is pretty straightforward. Brownell's "Gunsmith Kinks" describes the procedure.
You likely will get only one chance to repair it, so use the right materials and techniques!
Remove the barreled action and magazine from the stock.
If it is a partial crack, open it up and hold the crack open using toothpicks while you mix the adhesive.
If the crack is complete, or nearly so, go ahead and break it apart to access the surfaces needing adhesive.
Brownell's Accraglas (liquid, not the gel) two part epoxy is the only adhesive you should use. I've never had a cracked stock repair come back to haunt me after using Accraglas. The stuff is several times stronger than the wood it repairs.
Mix a suitable quantity according to directions, and apply to the joint surface using popsicle sticks or toothpicks. Poke and dribble it into the less accessible places, to make sure you have complete coverage. Use thick rubber bands, duct tape, and clamps to hold the crack together while waiting for the epoxy to set. Make sure the broken parts are aligned properly. Let it set up overnight, and use an exacto knife to scrape and remove excess adhesive. You may have to refinish the stock at this point. Reinstall the barreled action and magazine parts. Check for proper fit. Wait several days for the epoxy to fully cure before shooting.