Hi, did you hit something with the mower???
Sounds to me like you hit an immovable object and the keyway sheared and the motor kicked back taking the cord with it.
The starter is riveted to the housing but the housing is bolted to the mower 2 bolts front 2 bolts back. You need to strip off the tank (you dont need to dissconnect it you can just lay it over to one side as long as its not too full of fuel) and get the oil dipstick tube out of the way to get the cowling off the engine. Its easy to do the cord then.
Before putting it back together i would be undoing the nut that holds the starter cup on the top of the motor and checking the keyway. No point not doing that as if you put it togetehr and its sheared it will do the same again and it will try and take your hand with it. It hurts.
