If your starter motor is refusing to turn your engine over, first check the battery has sufficient charge and is in god condition. Next, scrutinise the ends of both battery leads, plus the engine-to-chassis-to-body earth straps. The wires can end up making very poor contact where they’re crimped into the terminals. This can due be down to salt and moisture ingress or vibration. If they seem in good order, you’ve narrowed the fault down to the motor or solenoid. Test as shown in the diagram.