If you accidentally delete mailbox and if you wanted to reconnect it back then you may not be able to find it Disconnected Mailbox. You have to  run Clean-MailboxDatabase to get the deleted mailbox. Also if you want to disconnect the mailbox to re-add it to an other user or the same user do the following:

• Disable the mailbox in EMC
• When you disable a mailbox the user object stays in AD and the mailbox is marked for deletion.
• The disconnected mailbox should appear in the disconnected mailbox view
• If it is not appearing in the disconnected mailbox view run one of the following commands from powershell

Clean-MailboxDatabase \servername\SGName\Store
Cleaning Database of Individual Store

Get-Mailboxdatabase | Clean-MailboxDatabase
Cleans all the database in the Organization

Get-Mailboxdatabase | Where{ $_.Server –eq “<servername>”}| clean-MailboxDatabase
Cleans all the database in the specific store

Get-Mailboxdaatabase | Where{ $_.Name –eq “<DatabaseName>”}| clean-MailboxDatabase
Cleans all the Database which matches the specific name given in Databasename

• After the command completes, check the event viewer for the following  event ID’s
  • Event ID 9531 – the clean mailboxdatabase process has begun
  • Event ID 9533 – a user does not exist in the directory or is not enabled for Exchange mail. This mailbox will be removed from mailbox store  in after the retention time has passed
  • Event ID 9535 – the process completes and lists that the mailbox was retained in the store
• Finally you should see it in the disconnected mailbox view and you can connect it to the same AD user or an other AD user.

This blog was based on smtpport25′s blog, http://smtpport25.wordpress.com/2009/04/22/deleted-mailbox-not-appearing-in-disconnected-mailbox-in-exchange-2007/

If you need to restore the mailbox because it is not retained in the mailbox store, see these great sites for restore guide using Recovery Storage Groups
http://www.petri.co.il/using_rsg_in_exchange_2007.html
http://www.msexchange.org/tutorials/Working-Recovery-Storage-Groups-Exchange-2007.html