Margi Moul says:
Photos are stored in iCloud and external drive. Not in Pictures folder.
That's it. A Photos Library can't work when it's in the iCloud drive. Move it back to Pictures. Cross your fingers that it hasn't been damaged. Here is the documentation:
Move your Photos library to save space on your Mac - Apple Support
It says:
You can't store your library on a storage device used for Time Machine backups. And to avoid possible data loss, don't store your library on a removable storage device like an SD card or USB flash drive, or on a device shared over your network or the internet, including over a cloud-based storage service.
Double click on the Library and see what happens.