To reactivate a deactivated user:
In the Admin Hub, go to the Users section
On the left hand side, use the filters to search for the user's name, and ensure 'deactivated' or deactivation requested' is ticked under filters
Once you have found the user, click 'edit' into their profile
Go to the last tab, '4. settings' and change the Status to 'active'
Watch the video below on how to reactivate a deactivated user step by step: