After spending loads of time trying to resolve this thumbnail issue for myself, none of the usual suggestions worked. I tried everything!
This is how I solved the no thumbnail issue and hopefully it will help someone else.
My OneDrive space is full. I suspect this is possibly what initially caused the thumbnail loading problems. What I didn't realize is that my Windows Desktop folder location was set as C:/Users/MyUserName/OneDrive/Desktop. My desktop was where all of my image
folders were. So all of my images/folders that weren't loading thumbnails were in that OneDrive folder! I then changed the default desktop folder location to C:/Desktop. Initially I dragged and dropped my image folders from the old OneDrive
folder to the new desktop location . Still no thumbnails! I then tried simply copying and pasting the entire image folder to the desktop. BOOM all thumbnails are now loaded! So I just selected all of the rest my image folders that had previously been
in the OneDrive folder and copied and pasted them. They are all loading correctly now.
Short Explanation: For me this was all done on the Desktop because that's where all of my image folders were. IF your OneDrive folder containing your images is another location I suspect this will work the same.
Try copying the image folder from the OneDrive folder location and pasting (not drag and drop! ) it into a different non OneDrive location and see if the thumbnails load.