You can make up a missed workout (just go to it through the “Plan” tab by scrolling along the bar at the top). Once you complete the missed workout, the dot on the calendar (on the day you complete it) will turn green — however, this will not restore your previous Streak value.
Not completing any workout on a scheduled training day will break your Streak — it will reset to day one, and there’s no way to restore it to the previous level.