So the Content menu page (admin/content) on a stable production site loaded, and the only menu item listed was Comments...yet, the admin menu in the left nav listed all the usual suspects. WHAT was this?!
I checked the database, and the system section seemed fine, as did the record count of other tables as compared to the staging version of the database.
I strolled on over to admin/modules, and lo and behold, the Site Building section of admin was missing Blocks, Modules, Views and more. Though again, the left nav seemed fine.
Until, that is, I actually clicked one of the menu items in the left nav that was absent from the page. Nothing. Each was a link pointing to the current page.
As it turns out, it was a memory problem. The hosting account, in which this was one site, was set for 128Mb, and the file still had that entry. Somehow, however, a 'switch was switched' that had the domains in the account looking for their ownphp.ini file instead of the one in the document root.
It is weird that Drupal just stopped listing entries instead of simply annoying me with a WSOD!