How do I unarchive an email? I right clicked an important email, and accidentally clicked "archive". I am not even sure what that's supposed to do, archiving sounds like it should keep it but the opposite happened. I was able to find it in ~/Library/Mail/.../Archive.mbox, but it doesn't appear in the Mail Application. Is it possible to get it "listed" again?
There should be an Archive mailbox in the sidebar on the right side of Mail.  Anything you archive should be there.  If you don't see it, close Mail and restart your machine.