I've dealt with that problem alot. If you lift the seat and follow the lever back to the top of the Hydro you will get a pretty good idea how that lever moves. I usually find dirt and gravel gets slug up there and binds the lever. I bought one of those spring claw part retrievers at a local auto parts store to fish the gravel out with. Keep it with the tractor. That and a good hosing off clears it out pretty good.
You probably have some smaller stones packed underneath the lever too, so try to clean them out.
You will notice a white switch down there with a button facing the front of the tractor. Thats the nuetral start switch. The lever pushes this in in nuetral and allows the tractor to start. Take a long screw driver or whatever and push the button back as you turn the key. Make sure your brakes are set as you are still in gear. This should get you to the hose to clean it out good.