What could be wrong if your car will not crank or start up?

First off, won't start AND won't crank is usually electrical, dead battery, poor connections at the battery or starter, bad starter ( Do you have lights and gauges? ) then that is very much the issue. Do the lights dim when you crank, do they go out altogether, does it start to crank and slowly stop, or does it crank ever so slowly? these are all battery or battery terminal issues, check your battery and clean those terminals to clean metal, check the thick red wire going to the starter, it needs to be mounted well on the starter and not grounding out on metal along the way. Batteries can go bad and still show 12 volts, this happens a lot in winter and summer, extremes in temperature will just kill a battery in a matter of hours so have it load tested.