Rust sometimes is the culprit. put the lug nuts back on but don't tighten them down completely. Usually 2 turn from seated is best. Then slowly drive the tractor in a circular motion and the side thrust on the wheel should break it loose. Once loose, jack it up and remove. If the driving doesn't loosen it, then you will be left with no choice but to break out the sledge hammer. With another person holding a very long piece of 2X4 against the rims edge, hit the 2X4 with some sharp wraps and hope that it breaks loose. You need to be careful that you don't damage the rim. The wheel should be off the ground when you do this. Sometimes it takes rotating the wheel and banging the rim at various locations to get it loose. If all that doesn't work, then it is time to get out the torch and heat the rim at the center axle hole. This should expand the steel enough that additional hammering will then break it loose. I once had one that was so badly stuck that it took a porter power hydraulic ram to break it loose. Don't know the size of the wheel and tire, but if it is filled or very tall, be careful that it doesn't fall on you. Keep the lug nuts threaded on till the stud is flush with the top of the nut for safety reasons.