Help

if it's anything like my Craftsman, there's a lever at the back of the tractor with a linch pin holding it in position. When in either the high or low position, the tractor can be pushed by hand, when in the other, it stays in gear. Try to rock it back and forth while working the lever; perhaps it just didn't seat properly.

i would suggest that you try and purge the trans

I would also check to see if the belt that powers the hydro is not broken, or popped of the pulleys.