Appearance > Menus > Custom menu missing.

Problem:
/wp-admin > Appearance > Menus > original custom menu I created is missing, however the menu structure is still in place.

Diagnostics so far:
~ deactivated all plugins one by one first.
~ switched to Twenty Twelve theme, problem still exists.
~ reset plugins folder via FTP.
~ tried to find custom menu name in MySQL.

I could try and add another custom menu, but since the original is still showing on the public side of the site, I’m thinking I’m going to do more harm.

Any help would be appreciated.
Mark