Motor continous spins on On position?

I have a John Deere 18 hp Kawasaki engine and when you switch on motor continously spins. It has new cylinoid on starter. It has computer board and ignition switch changed, it still continually spins without firing the coils.It spins without going to start position.One day it was running perfect , the next day all these problems.If you have had these problems please tell me what to do.