If your course is past its end date or closed, it will not appear in your Dashboard.
For Academic Courses, they will appear on your Dashboard until the term end date instead.
Please see your full list of courses, present and past via Courses (below Dashboard on the Global Navigation Menu).
Click All Courses on the sidebar that appears. Direct link to All Courses.
If you are an instructor of a closed course or a department representative/unit admin, you can add Teachers to closed courses via NUS Tools.