I had that issue with Win10 (I am now on Win11), and have zero idea of why that happened. I did all the internet searches, resetting folder settings, cache reset etc., and nothing worked.

I wasn't up to doing a complete new install of the OS. The last resort for me was K-Lite Codec Pack

I know that some don't care for it, have stated it might cause stability issues, etc., but I was at the end of the "rope". It was try it or reformat my drive and start over. I installed the "Basic", which did rebuild all the thumbnails.

I had no issues or problems outside of the occasional prompt to update the software, which I didn't.

But, I suggest you google search you problem and you will find dozens of videos on ways to reset the thumbnails first.

Go to any folder (this is for Win11 since you gave no info on which OS you are using), click on the 3 dots in the ribbon, Options, View tab, and look in the Advanced Settings window to make sure that in the Files and Folders< Always show icons, never thumbnails is unchecked. Click Apply and see if it changes. That's a "first step". There are others.