just one folder of saved mail in Apple Mail is missing

I use Apple Mail. After reading emails in by inbox, I trash ones I don't need and I save ones I do need in probably 20 folders. One of those folders is suddenly missing. I've searched discussion forums and found questions for users who have lost ALL folders, but that's not my problem. I see lots of my normal folders in their normal order. But one is missing. To show you what I mean, one mailbox used to be right here...

Ideas? How can I get it back? It's possible I'm missing another one or two that I haven't noticed yet, but right now I think it's just one. And the saved messages in the folders I have seem right.

thanks,

--JS