On the page Manage Adults > Adult Overview and you search for an account that has been auto archived you will notice that you are unable to select the check box on the right hand side >>
The checkbox is disabled with the hover text: “User has not been active in last 2 years.”
When an adult account has not logged in for 2+ years, their account will be auto archived. The easiest way to tell if an account has been archived manually, or auto archived, is to go to the user's profile, and scroll to the bottom. There you should see their history section. When you open this, it should show you the details here. This screenshot shows the detail as it will appear in their history section >>
Auto archived accounts cannot be unarchived, except by a System Administrator. Please get in touch with firstname.lastname@example.org who will be able to help with this.