Click on the name of the Organisation Unit in which the User sits from the Organisation and Users area of the site (see view an organisation unit if you're unsure how to do this).
Click on the Users tab.
Change the view to Archived by clicking on the Show dropdown menu.
Click on the Username.
Click on the Archive/Restore link within the Common Tasks area (you could also click on the Details tab followed by the Restore button if you prefer).
You will be taken to a two-step wizard.
Read the warning message.
Click on Restore.
The system will confirm that the User has been Restored.
Click on Finish.