Handy Hint 57 - How to get back your Inbox when it goes missing in Mail! Managing your Mailboxes.

This has happened to me, and to several other clients.  Somehow, we have managed to 'turn off' our Inbox in Mail so that it is no longer in the list of available Mailboxes!   Has it happened to you yet?  How do you get it (or any other missing mailbox) back