Have a 20 hp, riding mower. Was cutting grass the other day. Deck lowered to normal position, blades engaged, riding in 3rd gear. I felt a sharp "rap" under my left foot and the tractor stopped moving forward. Disengaged blades, raised deck, turned ignition off. I did not run over any objects (rocks, large sticks, etc.). Got back on tractor, started it, engaged blades, put it in 3rd gear, lowered deck, let off clutch and just sat there. Disengaged blades, shifted to reverse, let off clutch and just sat there. Any ideas what let loose?

Broken belt.

Hi,
I agree! You felt the end of the belt strike the frame. Give model number and you can probably be provided with all sorts of other info, like part number, diagrams and directions.
Peace,
Clarke