You say "deleted" but what you are describing sounds like the course is still there, it just doesn't appear on your dashboard as a favorite. A deleted course would completely disappear from the course list.
The How do I customize my Courses list as an instructor? lesson from the Canvas Instructor Guide explains how to make a course a favorite.
If you can still access the course by going to Courses > All Courses and scrolling down, then you can use that method. It is extra work, but it is designed that way to keep the dashboard from being cluttered with old courses.
If the course is listed under "prior enrollments" then you cannot make it a favorite to get it back on the dashboard. You may be able to fix this by going into the course Settings page and change the end date for the course. If your sandbox course has a term specified, then it may be that the term is over and so the course was concluded and removed from your dashboard for you. We set faculty access to our courses to end about two months after the semester is over to help them out so that they don't have to automatically remove the courses from their dashboard.
If you cannot change any of these settings, then you should need to talk with your local Canvas administrator.
This discussion post is outdated and has been archived. Please use the Community question forums and official documentation for the most current and accurate information.