It could be the battery and not the alternator. If you can get your car jump started, then it's the battery. Even though the lights and radio may work, your battery may just be weak enough where there isn't enough cold cranking amps left to turn your engine over. An alternator is NOT needed to crank your car over when your start it up. Even with a bad alternator, if you have a good battery, you should be able to start your car and have it running until the battery dies.
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