1) If the clutch suddenly stopped working (was fine five minutes ago, won't work now), you more than likely have a failed throw-out bearing. In this case, you split the tractor and renew the failed component (and probably a clutch set since you have it apart).
2) If the tractor has been sitting for 6 weeks and now the clutch won't disengage, then your clutch plate is more than likely seized to the flywheel. In this case (and you didn't hear it from me) check the tractor is freewheeling (not stuck to the ground), start in low gear and drive (uphill if possible) with the clutch in until it breaks free. Another method is someone pulls (jerks) the tractor with a tow strap while you have the clutch pushed in. (I know, shouldn't do this, dangerous, etc. - not suggesting you do it, just that I and many people have).
3) The other possibility is that by incorrectly adjusting the clutch (less than zero clearance) or / and, then stepping on it, you can bend the fingers on the pressure plate and lunch it. In this case, correction is the same as number 1.