Because the fly wheel contains the ring gear, you'll need to remove the flywheel to get to the ring gear.
Flywheels also contain the starter ring gear. Inspect the gear for damage. Most flywheel ring gears can be removed from the flywheel and a new one can be installed.
Inspect the ring gear for broken or missing teeth and damaged gears. Ring gear damage can be a result of a loose flywheel or a starter motor problem.