We need the model number off the serial number tag. The Quest series is over 20 different model configs. Basic Quest wiring is the same usually but not always.
What the OP is describing is safety switch failure. Most likely the seat switch assuming you still have the parking brake still engaged. The mower series runs 12v through the seat so it more prone to failure then one that simply ground things out. The seat, brake, PTO, and motion arms switches all controls the kill relay and fuel solenoid.
May simply be that the seat switch got unplugged.