You should have 12 volts on one small solenoid terminal when the key is activated and ground on the other. If you hear the solenoid activate, but the starter doesn't go- jumping the 2 large terminals on the solenoid should make the starter turn. If this is true, the solenoid is likely toast. The PTO sensing switch under the dash, brake switches must both be depressed to complete the starting circuit. If the engine is trying to turn, but can't- that is something else...