The muffler fell off.
A muffler falling off of a vehicle is usually the result of the hardware that keeps it fastened to the exhaust system failing or the muffler being rusted out near the flange that is bolted to the rest of the exhaust system and that has caused it to fall off. I would recommend enlisting the assistance of a certified mechanic, like one from YourMechanic, to come by and thoroughly inspect your Mustangs exhaust system in person. They will quickly be able to determine whether you need a simple muffler replacement or if more of the exhaust system needs attention. They will also be able to assist you in making the necessary repairs so your exhaust system can keep your Mustang sounding like it should.