To display mailboxes that are disconnected use the following command.
Get-MailboxStatistics -Server <server> | where { $_.DisconnectDate -ne $null } | select DisplayName,DisconnectDate
Replace <server> with the name of your mailbox server.
This gives a list of the disconnected mailboxes.
To reconnect you use the command below
Connect-mailbox –database <mailbox database name> –Identity <disconnected mailbox name> –User <User to connect to>
The command below gives list of disconnected mailboxes on a server called Server1
Get-MailboxStatistics -Server “Server1” | where { $_.DisconnectDate -ne $null } | select DisplayName,DisconnectDate
Ex-Shell 