Likely the reason your tractor will not move with foot pedal depressed is that the drive belts are stretched and worn so the transaxle pulley is not grabbing and turning enough to engage transmission gears. One or both belts could contribute to this problem. So it is best to replace both upper and lower drive belts unless you have recently replaced one of them. The long lower belt engages the variable speed pulley from the engine crankshaft pulley which spins in ratio speed to how much pressure is put on the belt from the foot pedal. The secondary upper belt runs around the variable speed pulley to the transaxle pulley that turns the transmission gears causing mower to drive. Wear/slippage from either belt can cause a loss of driving power, even little or no movement at all. Of course your belt may have slipped off of a pulley in particular a tensioner pulley. So first look under the tractor and make sure belt is on and routed properly and if necessary adjust or replace any belt keepers that may have bent or broken which can cause a belt to slip off a pulley when a spinning belt is suddenly relaxed from tension. Overly stretched belts also have a tendency to slip off. While you check the belt spin all tensioner pulleys to make sure the bearings are turning freely. Worn/noisy/ ruff turning bearings restrict/hinder belt movement and causes premature belt wear/burning.