If electric PTO the bearing in the blade belt drive pulley is probably froze up which would new PTO clutch assy is needed.
But before you replace make sure it is not a bad PTO switch causing the clutch engagement when you put the ignition switch in run position. Basically with mower not running listen for clutch as turn the switch to run from off. I haven't seen this but is an outside possibility.
One other thing to check for is make nothing got wedge into PTO clutch rotor/armature area like a stick or piece of the belt. This I have seen several over the years.
