I too have accidentally archived an email that I need back.  The problem is that I don't have an archive folder visible when I'm in my Inbox.  I see it only when I have an email open.  If I click on it then, it archives the email (at least I assume it does as the email disappears).  But I can't simply open the archive folder to retrieve anything.  Arrrgghh.  I hate the positioning of the archive, flag and delete icons immediately before an email subject.

How to I open the archive folder?