To undelete the deleted Archived mailbox, please follow the below steps
1. To find the MailboxID for the user run the below query against the archivemanager database changing xxxx to the display name of the user.
Select mailboxid from dbo.mailbox where name=' xxxx'
2. To undelete the mailbox, run the following query, where yyy is the MailboxID from step 1.
update dbo.mailbox set deleted=0 where mailboxid=yyy
