42 Gear model mower blades won't engage - Ariens 42" 19HP Lawn Tractor

You haven't said if its an electric clutch or manuel so I presume it is electric and controled by a switch(PTO) on the dash 2 reasons.
1 one of your kill switches is faulty example: seat switch which is in an inline circut with your PTO switch could be the problem or a fuse or a bad PTO switch, it usually is the seat switch but trace the wires down from the PTO switch
2 you could have a faulty electric clutch which is under the motor attached to the crank shaft. check the plug in,Does it engage ?sound of a large click and if it does that means the clutch has to be adjusted using a feeler guage.