When a users mailbox becomes disconnected it may not show in the EMS GUI. If the mailbox is not showing you need to use the EMS (Exchange Management Shell and enter the command Clean-MailboxDatabase <Database>, where database is the name of the database where the disconnected mailbox resides. In this particular case the command I used was Clean-MailboxDatabase “Mailbox Database”.

After this is run go back to the GUI and refresh and ALL missing disconnected mailboxes should appear. You can then re-connect in the usual manner.