Software :: Nested Mailbox Renamed - Individual Emails Lost Addresses

I renamed a Mailbox, trying to rationalize my boxes. The mailbox contained six other mailboxes, some of which have sub-mailboxes. i.e. three levels. A new (blue) folder appeared with the right name but nothing it. No folders, no email. I suppose the actual individual emails have lost their addresses or something. I went into a separate hard drive back up, into my User Library, and made a copy of the correct mailbox which has the email in it. I dragged it into Mail (I think?!) but it is uncoloured and although it has the nested mailboxes in it, there no email in any of them. How can I restore the email? I tried dragging the individual email but Mail wouldn't register it at all. I am very nervous about losing this email because it represents a vital part of the last four years of my research and I use it every day.