This might be a very rare case but it did happen to a number of student in a Moodle I support.

The course is in "Collapsed Topic" format. A student has the current section set to 8 according to the database, but there is no section 8 for the course, possibly because it was removed from the course. As a result, when the user tries to view the course, she see no content and just a drop-down list of other sections. Selecting a section makes no change to the page.

This was resolved by changing the current section setting to a valid section in the database. However, a solution on Moodle interface would be more preferred.