This could be caused by one of two things:
If the lesson is showing in the list of lessons but you do not have videos when you click, this is because the previous lesson is not marked as complete. Just go back to the previous lesson and click the button at the bottom of the page labeled "Mark as Complete".
If the lesson is not listed at all, then all you will need to do is fully log out of the site and log back in. Sometimes a web browser will cache in details of the page so the updated lesson will show as not available because the page isn't being re-authenticated with the new informatio