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. Try "jump starting the car" using jumper cables and another vehicle. It could be insufficient voltage in the battery (bad battery, have the battery charged and tested)., make sure the battery connections are clean tight and secure. Next I would check for power (12 volts) at the small wire at the starter when a friend turns over the ignition key to the start position. If there is power going to the starter from the ignition switch and the starter has battery voltage at the large wire at the starter, in this case the starter may be deffective.