The most common cause of thumbnails not displaying is a defective or incompatible monitor profile.
As troubleshooting, and as a possible temporary fix, try setting the monitor profile to sRGB. If you have a wide gamut monitor, use Adobe RGB instead.
With LrC closed, press the Windows key + R, then type colorcpl in the box. This will open the Color management dialog.
Add the sRGB (or Adobe RGB) profile, and set it as default.
If this fixes the issue, you should ideally calibrate your monitor with a hardware calibrator.
This will also create and install a custom monitor profile that will be more accurate than sRGB or Adobe RGB.
If changing the monitor profile doesn't help, you may have a corrupted preview cache, and the fix is to delete it.
With LrC closed, use the File explorer to go to the folder that contains your Lightroom catalog.
If you don't know where the folder is, go to Edit > Catalog settings > General, and click the Show button in the top right corner.
Delete the folder whose name ends with Previews.lrdata. Do not delete anything else.
The previews will rebuild as you browse folders in Library.
If this doesn't help either, please go to Help > System info in LrC, click the Copy button, and paste in a new reply.
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