WordPress Media Thumbnail Images Issue
There is a PlugIn titled Adaptive Images for WordPress. It works great, but...
If a site is ever cloned or upgraded, this issue could arise.
Images are gone, thumbnails, are gone, Media Folder isn't showing previews, etc.
Go to the Adaptive Images Settings, and click the Save settings Button. Everything should work again. Maybe it has to "Re-Adapt"
Also consider clearing Image Cache.
Ditch Adaptive Images. It isn't needed. No offense. In fact a compliment. Adaptive Images was ahead of the game. Before the HTML standard full embraced the idea of different size images for different devices with the SRCSET and SIZES Attributes, there was Adaptive Images. And well before WordPress fully adapted the the concept and made image resizing a built in function of WordPress, there was Adaptive Images. So again, no offense at all to Adaptive Images, the feature isn't necessary.