What's the difference between closing a class and setting the course offline?
If a course is set to offline, nobody except the course admins can access it. This means previously enrolled students can no longer access the course.
If a class is closed (while the course is still online), previously enrolled students can still access and complete the course, but no new students can enrol.
FullAMark Hi! To re-open a class, go to Course setup > General > Classes in your course > Change class access.