If you've already determined the PTO select lever is working properly, good chance the PTO friction disc is stuck to the flywheel. If so - and without breaking the tractor in half - you'll have to (try to) break it free under load. With the tractor in neutral and NOT running - and with the PTO lever engaged - turning the PTO spline may break it free. As to avoid damaging the spline teeth, this should probably be done by connecting a PTO shaft; apply the leverage to the shaft, letting it in turn try to rotate the spline.
Another method would be to connect a PTO implement, like a rotary cutter. Tractor should already be warmed up before trying this, then shut it down. Put the transmission in neutral, engage the PTO lever, foot off the clutch, start the engine, quickly bring up the RPMs. The act of lugging against the inert blades may also break the plate free of the flywheel.
//greg//