You will have to search for their name under the Advanced Search.

Once you click on their name, click Manage User.

Under the General Tab-Information, there should be a button at the bottom that says ‘Restore’, click that.

The user should be active again.

Just FYI….If the user has been permanently deleted, I don’t believe you will be able to reactivate them.

So William.L14, in response to what you are saying, this means you will have to add them as a new user to activate their usage.  I’m querying I’m unable to see replies, don’t know if I am doing something wrong.

If your users has been permanently deleted, they will need to be created as a new user again. 

Here’s an article on the Success Community about restoring archived users.

Restoring Archived Users

Hope this helps!

-Daniella