How do I get incoming mail in one of my mailboxes unstuck. It will not download any mail in one box but the other mailbox works fine

How do I get incoming mail in one of my mailboxes unstuck?? It is trying to download it but seems jammed. The other mailbox is working fine. I know there is mail there because it is coming into my bb. This just started this morning. I took someone's suggestion and duplicated my mail from the library and I then put com.apple.mail.plist on my desktop. I was unable to delete the incoming mail from the mailbox however so don't know what to do.

Help?