If a student has previously registered for an event and had their registration canceled but changes their mind and wishes to register again, you will need to reinstate the canceled registration. To do so, click Registrations and then Registration Cancellations from the left-side menu bar. You can then find the student and click the Reinstate icon. A secondary box will appear asking you to enter a reason for denying the cancellation request.
Once you enter a reason for denying the cancelation request, the Status for that particular student will update to show "Denied" instead of "Approved" as their cancelation request has now been denied. They will once again show on your Student List for the event.