Mountfield Sp536 Petrol Lawnmower (self Propelled Mechanism Not Working)

This Mountfield is no longer SELF PROPELLED. The rest of the lawnmower is in good working order and was bought in March 2013. It is otherwise in excellent condition and been kept in a garage and regularly serviced. My garden is large and on a slope and requires a self propelled unit. In good condition and working order except the self propelled mechanism no longer works..Excellent starter and runner. Specifications include:RM55 OHV Engine - 160ccAdjustable cutting heightAdjustable speed control51cm (21 ) cutting widthGrass collection bag.