What used to show on the screen as .jpg or .png photos, is now only the file name with no picture. How to fix this?

What happens, when you click the thumbnail with no picture?

Can Photos open the photo?? Can you use "File > Export > Export unmodified original" for this photo?

Or do you get an error message If there is an error message, what is it?

If you do not get an error message, try, if rotating the image back and forth with the shortcuts ⌘R and ⌥⌘R will bring the image back. Or try the command "Image > Revert to original".

If you get an error message about a missing original, try to find out how the originals may have got removed.

• The library could have become damaged by a cleaning application, that removes duplicate photos automatically.
• Photos may lose the connection to the originals, if the library is damaged by being stored in a synced folder (Dropbox or any cloud folder) or on a Network share or a volume with an incompatible file system format.
• Or, if the photos have not been copied into the library when you import them, like Russ suggested. Then we must not remove the originals files after importing them, because Photos needs them.

Could any of this apply?

It sounds as though the Photos library has lost the link with the photo file.

Are you using a referenced or managed library? You can check by opening preferences, is the box 'Copy Photos to library' ticked?

Glad you could solve it with the help of Apple Support. I think I misunderstood your question and thought you have been asking about the photo thumbnails in Photos or Mac, but you have been asking about the thumbnails in the Finder?