One of the plugs on a safety switch has fallen off or been broken or the switch itself is broken .
Prime candidate will be the brake switch which is under the mower .
To start off the key the power goes through the PTO switch then the brake switch then to the trigger on the solenoid .
The safety switches are ground switches
PTO + Seat
o r
Seat + brake
These are parallel circuits
And what matters more is weather it has a manual or electric PTO .