Explorer Window Folder Thumbnails Disappearing?

This started suddenly after I turned my desktop pc off before going away for a week. When I came home and turned it back on, it hasn't been "normal" since.When I start my computer, go to my user folder, documents etc., all appears fine. Folders/thumbnails and images within the folder appear as normal.However, if I open (for instance) my "downloads" folder, the folder icons and associated icons are initially displayed as "normal", but I see that the explorer bar (where I see C:/user/downloads, etc) is "computing items" but it never stops. A green "progress bar" SLOWLY moves across that "text window" and then it stays green. As if the pc is having trouble reading what is in the folder.Once that happens, if I back out of the folder and then go back in to it, most of the folder icons/thumbnails/pictures are no longer visible. If I navigate to other folders, it's the same thing; many (but not all) of the folder icons and the thumbnails within are not displayed. Example: in my "user file" the AppData and Tracing folders, the icon is non existant....yet my Contacts, Desktop, Favorites, Documents etc. are shown as standard windows "yellow folders" but not the thumbnails that I have associated with them. Also, the file info (if I highlight a file name) does not appear at the bottom of the explorer window, as it normally would for me.I have run my anti-virus, malwarebytes, which are showing no problems and I have also tried "indexing" but no luck. I do have an external drive that is close to full, but it is disconnected. My C drive is
currently 873gb full of 1.35tb. Recovery partition is 1.63gb free of 13.2gb - which I have never "messed with", though I have previously done 2 "reset to factory settings"