The folder structure is stored in a file named HIERARCH.PM which is located in your home mailbox directory.  Restoring a backup of that file will restore the folder structure to that point in time. 

If no backup, it must be rebuilt manually.

How do I restore the folder structure?

You need to find a backup of your hierarchy file. Mine is in the MAIL folder on my hard disk. It is called HIERARCH.PM

Hope that helps.

Lydia