Are you hearing a click-click-click followed by horrible screeching when you turn your key? You probably need to replace your starter motor. When functioning properly, current from the main battery is applied to the starter motor’s solenoid, and the solenoid engages a lever that pushes out the drive pinion on the starter driveshaft to mesh the pinion with the starter ring gear on the engine flywheel. A lot going on for sure. If this process breaks down, you aren’t going anywhere without a hefty jump-start or a new starter motor. The most common reason for starter failure is electrical corrosion on the contacts, and internal motor brushes.