There should be several safety switches on your machine. 1 for the blades, 1 for the seat, and 1 for the transmission. Sounds to me like it is not recognizing the fact that there is a person on the seat. Take a look and make sure the wires have not come unplugged from the seat switch. While at it, check the connections on the other switches. I have included a couple of schematics that should be close to your machine. Gives you an idea of where to look.