Question on hitting player already OB

[chuck chopper]

I'm just a football fan not an Official. This past weekend one of the games had a penalty on a player who was knocked OB then came back in and was 1st to touch the ball.
Announcers were talking about possibility that he was hit while OB & prevented from coming back in. What is Penalty ?
In this case it happened on a 4th down punt and the player involved on the kicking team got knocked OB then came back in (10 yards further down the field) and stopped the ball from going into the endzone. Officials ruled a touch-back because of him being 1st to touch ball. 2nd question would be is he req'd to come back in where he got knocked out ?