Most of the mowers are equipped with an electric clutch. Look under the engine area, on the end of the crankshaft up near the engine, and see if it has an electric clutch with wires coming out of it. They do fail! RJ

OK, can we assume you have checked to see if the deck drive belt is still present?
There is an operator presence relay on your tractor that controls the ground side of the PTO clutch.
This is a dual function relay in that it can kill the engine under one condition of the seat being vacant, and can deprive the PTO clutch of its needed ground under a different condition of the seat being vacant.
The click you hear, is it distinctly audible with the engine running? If so, it certainly seems like the PTO clutch is operating, hence my question of whether the deck drive belt has broken or otherwise come off a pulley.
We can certainly guide you through some electrical checks but the click sound makes it seem like the PTO clutch is operating.
No point in running through several tests only to discover a belt issue afterward.