Sounds like you just lost a shearbolt. That's common and nothing to be alarmed about. I would tighten the bolts and check the 80W lube oil in the gearbox. Also, raise the cutter and look underneath. Make sure the stump jumper pan is tight and not wobbling on the shaft from the gearbox and also check the blade bolts for tightness. The will move up-down some, but not much. Just make sure everything is tight and replace your shearbolt with a 1/2" dia. 4" long grade 2 bolt that you can buy from the bulk bins at Tractor Supply or most any other hardware store. Make sure you use grade 2. You do not want a grade 5 or grade 8. The grade 2 will have a smooth head with no marks. The grade 5 will have 3 marks and the grade 8 will have 6 marks on the head. Us only the ones with no marks on the heads. You will probably have to rotate the shaft until the holes align and then drive out the old bolt from the center shaft with a punch. The shear bolt is there to protect your tractor and gearbox if you hit a rock. It shears off and keeps other stuff from breaking.