I have an Oliver 550 tractor.  Today, as I was cutting with a bush hog, I disengaged the power take-off clutch lever.  When I did so, I felt a small amount of grinding and then the power take-off clutch lever moved freely forward and backward without any engagement of the PTO (the PTO no longer turns nor can it be engaged).  The power take-off clutch lever mechanism seems to be working properly all the way to its insertion into the tractor body.  What happened and how do I repair it?