To resume drawing from where you left off with the Pen Tool just click over an endpoint and drag to set a smooth point handle.
Very likely this will make an anchor point with only 1 handle rather than the 2 you want for a smooth point.
The Anchor Point Tool can be used to change a corner point to a smooth point- just click and drag with it on the point you want to convert. It can also change a smooth point to a corner point by simply clicking on the point. It can be used anytime while drawing a Path or after the path has already been created.
It can be accessed from the Pen Tool stack on the Toolbar or by holding the Alt/ Option key while using the Pen Tool or by pressing Shift+C from any tool.