When you archive a course, you have simply removed it from the courses you are currently learning. This means that the course is still there for you to pick up whenever you feel like it. Think of it as putting it in storage in your garage or something like that.
To recover it, simply head over to the archived courses page here:
Alternatively, you can search for the course in the shop. Once found, on the course homepage, you can simply click "Get Course". Once clicked, you will have recovered the course and it will once again appear on your home page.