As mentioned earlier check linkage first and also check adjustment. It is usually that the clutch material has rusted onto the flywheel. You can try to release it by moving the tractor to an open place where you have lots of room. With the tractor off, put it in 2nd or 3rd gear low range. Start the tractor in gear and while depressing the clutch briefly stomp on the brake and release it again and repeat, all while depressing the clutch pedal. This will usually remedy the problem. Sometimes you might have to start in a higher gear to enact more force on the clutch and flywheel. Be sure that if this does not work the first time that you leave yourself enough room to safely shut down the tractor before hitting something or driving off a cliff..
You can also try using a ground engaging implement like a plough to do a similar thing by starting in gear and lowering the implement into the ground and depressing the clutch.
Hope this helps!!