Try the following in order as necessary:
1 - reformat the card in the camera, take some test photos and see if it is recognized by Photos.
2 - As a test launch Photos with the Option key held down and create a new, test library. Test the memory card with the new library. Does it work? If so open your main library and test the memory card again. This tells us if the problem is limited to your current library or is more wide spread.
3 - Log into a new, basic, admin user account, Set up users, guests, and groups on Mac, and repeat the test in #2. This tells us if the problem is limited to your user account or is system wide.
4 - Lastly, boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists with your main library and the test library. Reboot normally and test again.
NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following;
• Verifies your startup disk and attempts to repair directory issues, if needed
• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)
• Prevents Startup Items and Login Items from opening automatically
• Disables user-installed fonts
• Deletes font caches, kernel cache, and other system cache files
