Engine Transmission stuck in drive

[aqua-holic2000]

Have a Scots riding mower and when mowing years ago I heard a pop and transmission was stuck in drive. Been sitting ever since. Any thoughts?

 

[bertsmobile1]

If it has a manual gearbox then one of the keys on the selector forks could have broken
But there were a lot of Scotts mowers & because you provided no information this is just a wild guess