Your mechanical PTO handle operates a lever down on your deck via a cable. The cable is attached to the deck and hooked to a spring.
When you move the PTO handle to "engage", it pulls the cable and spring which tightens the deck belt mechanism to engage the blades.
Obviously some mechanical aspect of the process is amiss. YOU have to look at it, operate it, and figure it out.
Since you have no tension you could have a broken cable, a broken spring, a broken belt, or something came loose or disconnected.
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