Most sit on mowers and riding tractors have an "operator present" switch beneath the seat. If this switch is not activated then engaging the blade will cause motor shutdown - which is what you want if you fall off the mower or even stand up off the seat. The switch could be disconnected, defective, loose or messed up with dirt. Be sure this part is working and I'd expect you will have it fixed.
Answers & Comments
Most sit on mowers and riding tractors have an "operator present" switch beneath the seat. If this switch is not activated then engaging the blade will cause motor shutdown - which is what you want if you fall off the mower or even stand up off the seat. The switch could be disconnected, defective, loose or messed up with dirt. Be sure this part is working and I'd expect you will have it fixed.