Just as Items do, Activities have a status. Activities can be published, unpublished, and archived.
Activities must be published in order for a student to see it. If a student tries to view an unpublished Activity, there will be an error on the page and they won't be able to see it.
A published Activity is one that is ready for student participation and is visible when accessed by the student.
An archived Activity is deleted, but can still be accessed by searching by the Archived status in your Activity bank. You can then access the Activity and change the status back to published or unpublished if you wish.
Change the Activity status using the status drop-down menu, located in the details section of the Activity Editor: