photos cannot find system photo library named photoslibrary
Everything was working fine until yesterday. I dowloaded photos from my camera, into Photos app on my Mac, which didn't back up to iCloud so I could see them on my iPad and iPhone. Today, Photos app on my Mac won't open at all. The dialogue box says "Photos cannot find the System Photo Library named "Photos Library.photoslibrary"
I have no idea how to fix this. Please can someone give me some guidance. Can I get my iCloud backup to replace the library?
Mac mini, macOS 10.13