So there are many things that can cause this... But being it happened while you were moving brush I would check the neutral safety switch on the transmission.
This switch is on the left side of the transmission housing. You should be able to see it by looking up under the platform. The switch is internally actuated from inside the transmission, but it looks like a round cylinder with two leads coming out of it & it has a hex head at its base. Look for the two wire leads that are attached to it & make sure they are still connected.