In American football what happens if you call a fair catch during a kickoff and you don't catch it?

A player who signals for a fair catch is not required to catch the ball. However, if a player signals for a fair catch, he may not block or initiate contact with any player on the kicking team until the ball touches a player. Penalty: snap 15 yards behind spot of foul.

https://en.wikipedia.org/wiki/Fair_catch#Penalties

If a fair catch is called on a kick-off and not caught, and the kick travelled at least 10 yards, the ball remains live and may be recovered and advanced by the kicking team, making it VERY important to catch the ball. On a punt, the ball is not considered live until it is touched by a member of the receiving team, so allowing a ball to hit the turf is not as critical.