When you delete a user, it's more like you've put their account into hibernation. While you won't see them on reports or in the learner list, their records are maintained, and they can be restored at any time. This is actually very simple.
To get started, we'll need to get to the Manage Users screen.
First, navigate to the Admin section using the Application Switcher in the upper right. From there, click on Users & Permissions  and in the menu that appears, click Manage Users .
Next, click on Filters , then on Terminated . This will show all of the users you or other admins have deleted. Find the user you want to restore, and click their name.
On the next screen, simply click Restore in the upper right. This will restore a user with all of their records intact, and they can log in with their old password. Alternately, you can send them a password reset email to get them back up and running.