A broken drive belt, bad transmission or problem with the clutch cable can prevent a tiller's wheels and tines from turning.
Check the drive belt and replace it if it's worn or broken.
If the belt is okay, check the clutch cable to see if it tensions the belt to spin the transmission pulley when you pull the bail arm up against the handle. Adjust the clutch cable if needed or replace the clutch cable if it's broken.
If the transmission pulley spins but the tines and wheels won't move, then you'll likely need to replace the transmission.