1. There might be a change in the "View". To confirm that, Open the folder that you are having problem >> Click on the drop down arrow on "Change View" button at top right corner of the folder >> Make sure that either "Large" or "Extra large" were selected.
2. You need to go to folder options. Open any folder >> Click on "Organize" >> Click on "Folder and search options" >> Go to "View" tab >> Uncheck the box beside the option "Always show icons, never thumbnails"(The first option in the "View" tab) >> Click on "OK".
This will fix the thumbnail view. If you are still having the issue, go to next step
3. Click on "Start" button >> Type "Cleanmgr.exe" in the search bar >> Press "Enter key" >> Click on "OK"
4. You will get Disk Cleanup window that says "Calculating." and then the Disk Cleanup options screen.