Have you checked the following:
*You are in Neutral and the PTO is off.
*Make sure the HST treadle is not being pressed in either direction and it has returned.
*Bring the battery to an auto parts store and have it load tested.
*Check all fuses, use a digital multimeter if you aren't sure and check continuity.
*Check all wiring to make sure nothing is disconnected, cut, or torn up (check for squirrel, chipmunk, mouse, or rat infestations).
*Check the ignition key switch for proper operation using a meter.
I'm betting on a dead battery or a lever that isn't in the correct position.