Have you been using iCloud Photos? And perhaps "Optimize Mac Storage"?
And how did you export the photos from Photos? Did you export the unmodified originals or the edited versions?
Generally, you will lose either quality or your work, when you rebuild the library from the exported photos. YOu will lose the albums, folders, projects.
- If you saved only the originals you will lose all adjustments you applied and the metadata you added, unless you exported originals with sidecar files to save the metadata.
- If you exported the edited versions, you can no longer revert to the originals. The quality will be less.
The best option is to copy the Photos library back. It will have saved the originals and the edited version paired, and you will get all albums , projects, folders back.
If you have been using iCloud Photos, and have been using optimize storage, the saved library may not be complete. The optimized, incomplete versions of the photos and videos will be removed from the library and you should enable iCloud photos again, so the missing originals can be restored from iCloud.
If your Photos Library is currently in iCloud Photos, I would simply start over with a new, empty Photos Library. Make this empty library your System Photos Library and enable iCloud. This will recreate a fresh copy of your Photos Library. Only the projects will be missing.
