I am not sure of your setup but many systems ground out the ignition to shut them off. Follow all wires from ignition switch and trace each one to make sure none are grounded out ( melted rubbed etc). One wire will go to ground; disconnect it and try to start it. Also make sure all safety switches are working ok ( seat switch, pto switch etc. )Those all ground out ignition. Since it happened suddenly it could be a safety switch or a wire rubbed through or melted. Could be a coil or depending on age, points or a condenser.