The PTO is wired into the safety circuit so you can not get off the mower while the blades are spinning and put your hand under the deck to clear out a blockage.
So yes you are most likely correct, it is not getting power because one of the saftey interlocks or the wires between them is open circuit.
On most mower the PTO power has to go through all the interlocks before it gets to the switch.
So you now need to get your magnifying glass out, fill the pipe, pop the deerstalker cap on and do some investigating while muttering "elementry deer Watson elementary ".
A circuit diagram will be very useful but they are hard to come by to prevent the types of idiots mentioned in the first line bypassing the safety interlocks.
Personally I believe in evolution so am happy to let idiots to kill them selves but the Government believes in creationism so they can legislate out stupidity.