This just happened to me, too. Oddly, I shared a folder with a ton of files inside it. The folder has disappeared completely from the web interface, BUT, it shows up in the "Shared - Dropbox" list. Oddly, even though I created the folder and it's mine and was not shared with anyone else before, it now shows "You haven't joined this folder yet" underneath the folder name.
If I click "Join" I get an error.
I waited 10 minutes and tried again. Boom, the folder is back!
It seems when you share large folders, Dropbox does something in the background and the folder temporarily disappears. If you wait long enough, it re-appears.