My car turns over but after I drive it for a while and turn it off it won't

It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. If the engine will not crank over at all once the car has had a longer drive and the engine has reached operating temperature perhaps the stater is "heat soaking" causing it's failure, once it cools down it restarts. Check the battery connections and battery ground are clean tight and secure. See that the headlights are bright when the engine will not crank over and using a voltmeter check for voltage upon cranking at the starter solenoid.