All my mailboxes have disappeared!

By using the "Mailbox" menu I can go to "all inboxes" and I can also move to "next" mailbox or "previous" mailbox, but I used to be able to see a full list of all my mailboxes (my filing system!) on the left hand side of the screen and now I can only see the mailbox I am currently in. How can I get the full list back?