unlikely the wiring. AC electricity usually won't run a motor backwards. pull the motor away from the hand wheel and hopefully you will see it spinning the correct direction. And BTW if you leave this machine idle for more than a week you might want to put half a clothes pin between the motor and the side. The friction wheels can develop flat spots. I am guessing your problem is in the stich length control. I don't have one near right now though.
And welcome aboard.