I bumped a tree with the mower and now the Rear-end won't drive the mower

It's the kind with the rubber wheel that rolls agains the belt driven Plate.

Its not the rubber wheel, I can turn it by hand and the wheels start to move the mower then starts slipping.

Is there a Shear Pin somewhere and if so where?

Or what else can it be and how do I get to the dumb thing?

Thanks in Advance

Tinhorn