In Exchange Management Shell (Note that this is not the same as the Windows PowerShell), enter the following command:
Where MailboxDBName is the name of the mailbox database.
Then, in Exchange Management Console, go to Disconnected Mailboxes, right click the mailbox and click connect. You will then be able to select a user that you’d like to associate the recovered mailbox to.
If the users mailbox isn’t there, you will have to restore the mailbox using an item level restore into Exchange.