Like "show in Windows Explorer." I can google "how to show all folders in Windows Explorer" so I take it that you tried that.
I also can use the command DIR /A and I see almost all folders, hidden or not.
The folder still shows - with content - in
C:\Users\Username\AppData\Local\Microsoft\Windows Live Mail\Storage Folders\subfolder1\subfolder2\subfolder3, but no longer shows in the Windows Live Mail program. A different subfolder (a 'sibling') still shows. How to make it show again ? Win10 Pro v1511