First, go to your People page: Click EMPLOYEES in the menu at the left side of the screen.

Click + Add a filter, then choose "Show Archived" at the bottom of the list.

Once you scroll or further filter the list of people to find the person you want to unarchive (their name will be a lighter color of gray), click their row to open the details panel.

In the details panel, click the Actions button and in the dropdown, select Unarchive.

Their account will be restored, but no other actions will be taken (no email notification).