If it was a loud bang like a back fire it probably was a backfire and may or may not have sheared a key. When a flywheel breaks loose is sounds a lot like a 22 rifle shot, sort of a very quick loud "CRACK" sound if you could hear it over the sound of the engine. I suppose other things might have caused such a noise also. If you dealer is handy
call him and make it his project to check and repair. You can unscrew the timing plug, put it in 5th gear with brakes applied and try to loosen the flywheel nut with a torque wrench. If it reads at least 47 pounds, youi might loosen it and retighten to 47# and try to start it again. The safest thing other than dealer repair is to pull the cover and flywheel and check out the key and rotor fit. Probably should order a spare woodruff key just in case.