If the wheels on your tiller don't turn but the tines do, you may need to adjust the clutch cable or replace the transmission.
If the tines turn when tilling soil but the wheels don't move, you'll likely need to replace the transmission because the tiller clutch cable is properly tensioning the drive belt to spin the transmission pulley but the transmission isn't driving the wheels.
If the tines spin when they're above the soil but stop when you lower the tines into the soil, then you may need to adjust the clutch cable so it properly tensions the belt to engage the transmission pulley. Replace the clutch cable if it's damaged.