On field goal plays, once the snap is made, the ball is considered to be in play no matter what. That makes it possible for the defense to somehow recuperate the ball and score a touchdown. For that, a field goal kick must either be blocked by the defense or fall to the ground before the goal post. If either thing happens and the ball ends up with the defense, they can run the field and score a touchdown. A touchdown scored off a failed field goal attempt is a regular touchdown, and hence it is worth six points, plus the chance to score one or two more.
If an extra point conversion is blocked and returned, it is worth two points, as it is not considered to be a touchdown. A blocked field goal return is rare in football of all levels. There are many things that have to go right for a team to block the kick and still end up with the ball in a position that allows them to run the whole field and score.