Use Disk Cleanup to remove your thumbnails
Your thumbnails use thumbnail cache, but if the thumbnail cache is corrupted you might experience this problem. To fix the issue, you need to remove the thumbnail cache and Windows will recreate it. This is a simple process and you can do it by following these steps:
- Press Windows Key + S and enter disk cleanup. Select Disk Cleanup from the menu. Alternatively, you can just open the Start Menu and type disk cleanup to search for it.
- Once Disk Cleanup starts you’ll be asked to select the drive you wish to scan. Select your System drive, by default it should be C, and click on OK.
- Wait while Disk Cleanup scans your drive.
- When Disk Cleanup window appears, check Thumbnails from the list and click on OK. Few users are suggesting to select Temporary files as well, so you might want to do that.
Windows will now remove the thumbnail cache from your PC. Once the thumbnail cache is removed, Windows will generate it again and your problem should be resolved completely.
There are 21 different ways to fix this, No. 7 is working for me, so I copied it here. This is the source URL: https://windowsreport.com/thumbnail-previews-not-showing/
Also other reference:
https://www.wikihow.com/Enable-Image-Preview-to-Display-Pictures-in-a-Folder-(Windows-10)#step_2_1
https://www.thewindowsclub.com/thumbnail-previews-not-showing-windows