I am assuming you have a gear drive (9 speed) tractor which has a 2 position clutch. Half way down stops forward motion and fully depressed disengages the pto(s). It is possible for the pto clutch to be rusted or stuck to the plate and not disengaging (lack of use). A possible cure would be to attach a pto driven implement (rear or mid), engage the pto with the tractor OFF. depress the clutch fully and start the tractor. You will probably have to override the safety circuits with a jumper to start with the pto engaged. good luck!