Sorry for the confusion, and happy to be of assistance!
The first thing to do would be select
Perspectives -> All items
from the menu bar. Is the folder in question still missing?
If it appears at this point, the folder wasn't gone, it was just being hidden by whatever settings had previously been chosen under the View menu.
If so, then the next thing I would do would be to check under the File menu and see if the "Open Archive" menu item was active. (If you're syncing between multiple macs, check them all.)
If that menu item is active on any of your machines, your completed actions got moved to a second database on that machine. If you open the archive, is your missing folder in the window that appears?
If neither the "all items" perspective nor a search of the archives turns the folder up, then it sounds like it was deleted. In that case, open up OmniFocus' preferences. In the General pane, you'll see a folder location where the app stores backups of your data. (By default, the location is in an "OmniFocus Backups" folder in your Documents folder.)
Open that folder in the Finder and you can bring up your database backups. If you work backwards, you can locate the most recent backup that has the Archive folder. Drag it from the backup window and drop it into the 'real' database window and you've got everything back.
That help at all?