I have a 2001 Chevy

it could be the clutch if ur clutch is not disengageing properly it wont let u and how old is the gear box u may find it is that if its old or u may also need to check the gearbox oil if that has not been changed in years since u brought the tractor it may have grit and dirt stoppin the gears movin perfectly, personaly i think ur gear box is nakered