How Do I Repair My Photo Library In The OS X Photos App?
If you are experiencing issues like crashes, missing thumbnails, failed imports, missing photos, etc. you can try repairing your Photo Library. Before getting started, it's best to backup your Mac as a precaution. Once that's done, quit the Photos app on your Mac. Then restart the Photos app and hold down Command and Option while it's launching. A dialog will appear giving you a choice between "Repair" and "Quit". Proceed with the repair (it might take a while) and hopefully your issues will be resolved after it finishes.