To restore a Staff or Contractor user, navigate to the Staff section of your site then choose either Staff or Contractors depending on the type of Contact that you wish to restore. Once on the appropriate page, click the "Restore Staff" button.

Staff:

You will then be presented with a pop-up window which shows the deleted Contacts available to restore. Simply check the box next to the name of the Contact that you wish to restore, and then click the "Restore" button: