If you have NO idea how to diagnose the problem, have it looked at before returning it. If you have it inspected, at least you'll know WHAT'S wrong with it, instead of returning it, just to find out that you blew a 20 cent fuse. It could be one of a LOT of different issues that would stop your gun from functioning: fuse, motor leads, battery, trigger contacts, wire short, etc. I don't know of a place that wouldn't at least inspect the basic external problems for free; just make sure you bring everything with you. Hope that helps.
-BATMAN