The number one cause for self-propelled lawn mowers not to move is the belt, it may have broke, is worn, or come off the pulley. If it is broke or worn it would have to be replaced. If you check and the belt is in good shape and on the pulley correctly, the next thing to check is the drive cable. The drive cable is known to wear after time and it can become brittle and break. It may have also just unhooked itself from the self-propel mechanism on the lawn mower, if this is the case it can just be reattached. If the cable is worn or broken it should be replaced.