Open Photos to see your photos in this list.

Boot into Safe Mode (Use Safe Mode to isolate issues with your Mac and Playing Safe: what does Safe Mode Do?) and check there to see if the problem persists. (This tells us if there's any 3rd party software that might be contributing to the problem.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

You may need to make your Photos Library the System Photo Library. After you launch Photos, check the "Photos Preferences > General". If the button "Use as System Photo Library" is not greyed out, click it.

If you are using iCloud Photos, you may need to download the photos from iCloud, if your iCloud Photos Library is using "Optimize Mac Storage". View the photos enlarged in Photos.

Did you try to open Photos? You probably don't have to leave it open, just open it, wait a few minutes and close it.

It would be nice if you told us what you had already tried.