Go to the passenger side where the steps are, kneel down and look to towards the rear axle. There are 4 (steel) cables going to the rear axle. The top 2 going to a leaver on the side of the housing, The second one from the top is your gear shift cable. Maybe there is dirt, rock or something jamming the leaver and lube it. The steel cables are similar to bicycle brake cables, just much beefier. I had those break before, but not stuck. Before you take it part, just drive the tractor and try to take it in and out of gear without using the clutch, while accelerating or taking the throttle away fast. Kinda full throttle followed by no throttle making the tractor jerk around. Hopefully she will brake free...