Hi mikey,
From what it sounds I think you may have a sheared keyway. When the gear box was last driving did it make any noises? Firstly I would check all the linkages are still in tip top condition as I have found quite a few problem with split pin missing/bent.
Also is it the standard mtd gear box that is just forward reverse on gears and speed on belts? Ie: one control for forward and reverse and one for various speeds.
Hope you solve the problem quickly,
Kindest regards jay
