System Photo Library

Hi

Is it still in the recycle bin? If so recover it from there. If you've removed it from the bin then you'll have to recover from a backup. Time machine as OT said if you use it, or whatever other backups you have.

Alternatively if your library was synced with iCloud then you should be able to recover it from iCloud by creating a new empty library, and syncing that with iCloud.

Go into Time Machine and go back to just before you deleted the library, select it and restore.