John Deere 68 - runs great, doesnt go anywhere

Hi there! I picked up a JD 68 RER a few years ago to mow my sloped lawn. Works great - or it did. Last week, after mowing the side, I pull around to the back and stopped to close the gate behind me. When I got back on and put it in gear, it wouldn't go anywhere!

gearbox "feels" normal

blade still turns

engine runs great

parking brake non-functional NOW, but worked prior to this incident.

i peaked inside at the chain and it appears to be fine.

I'm mechanically inclined, but not much experienced with lawn-mowers. Can someone(s) recommend the steps to take to get this scooter back to work?

My manual states its for serial numbers 95,001+

Thanks for your time!
Jeff