Excel xls and xlsx files no longer preview

I used to get previews for Excel xls and xlsx files. This stopped happening and now I just get a Shell Icon image instead. How can I fix?

If you open the preview pane in Windows Explorer does it show Excel previews?

Yes. The preview pane in Windows Explorer will display an Excel file. Windows 10.

Okay, so as of this morning the OPUS preview window is showing Excel files. Perhaps the reboot solved the problem. Thank you.