my engine died yesterday en route to work. the car has had no trouble or repair in ages. it died when i came to a stop at a red light. it started right back up easily, but died as i shifted into drive. a coworker drove up and pushed me the last two blocks to work, where it now is resting. it starts easily and as many times as you twist the key to start it, but dies within a second of being shifted into reverse or any forward gear. what are the main possible causes (listed in order of probability if possible?)