What is wrong when car runs and then will not crank?

Battery cables could be loose, battery could be bad, starter could be bad, starter solenoid could be bad, ignition switch could be bad. Turn on the headlights and try to crank the engine. If the headlights dim or go out look at the cables or battery. If they stay bright look at starter, solenoid or ignition switch. The neutral safety switch or clutch petal switch could do it too.