If nothing in the car is working at all (radio, lights, etc.) then it could be a serious battery wire issue. If these do work, but the engine won't turn over, then it might be the fault of the starter motor or it could be some kind of blown fuse/relay. Check everything electrical out - especially the power distribution box under the hood. If everything looks to be in order (no mouse-chewed wires), take it to a mechanic and he should be able to get it going. As far as cost, it depends wholly on the type and extent of the problem.