I agree with rbargeron, very uncommon for the PTO clutch to just go out.
I would exhaust all other sources of non operation first.
Most common is PTO engagement lever is not moving all the way to engage.
second most common is internal damage to the PTO drive.
The PTO clutch is either the same clutch as the tractors drive (single disk clutch) or a second disk off of the first disk (dual disk clutch).
Most commonly the second stage becomes stuck from lack of use and sees very little use normally.
If you get the WSM for that model it will break down all the required steps for testing and if needed the replacement of the clutch, or any other steps and procedures needed to repair it.
If it is a dual disk clutch, you must follow the repair procedure and adjustment to the tee, as they can be fairly tricky to setup and get right.