The simplest test is the whumping the starter with the hammer test. What happens with electric motors is that they little carbon pads that make an electrical connection with little copper pads on the shaft of the motor. This intermittently energizes one of multiple sets of coils of wire inside the motor that reacts against a magnet. After much use, the carbon pads and the copper pads wear down and don't always land together making contact. This is probably what happened to you. By whacking the motor, you can possibly cause it to make contact again and work. This is not a fix, but rather a "get you home" method.

Good luck!