Follow these steps to use the Photos library repair tool:
- Hold down Option-Command while you open Photos.
- In the dialog that appears, click Repair to start the repair process.
You may be asked to type in your user account password to authorize the library repair.
The repair tool will analyze the the library's database and repair any inconsistencies it detects. Depending on the size of your library, this may take some time. When the process is completed, Photos will open the library.
If you repair a library that's automatically updated with iCloud Photo Library, the entire contents of the library will re-update with iCloud after the repair process completes.