Missing Photo Library in Photos App on Mac with iCloud Photos

Probably you accidentally made a new empty Library. The original one should be in your Pictures folder. You can double click on it there, or you can open the Library picker -- close Photos, option-click on the Photos icon:

You get a list of all the Libraries that Photos knows about, and it shows the path to them. You original one should say "System Photo Library."

Where did you keep your Photos Library?

Photos on macOS 15 cannot open a Photos Library, if it is in an unsupported location.

For example, if you kept your Photos Library on your Desktop or in the Documents folder and turned on iCloud Drive for Desktop and Documents, Photos will no longer be able to open your Photos Library, if it is now stored in a folder on iCloud Drive. Also, your Photos Library must not be on an external drive, if the file system format is not MacOS Extended (Journaled) or APFS. See: Move your Photos library to save space on your Mac - Apple Support

On the other hand, since all your photos are in iCloud Photos, you need only to enable iCloud Photos again for the new, empty Photos Library you are having now. It will be faster than reconnecting your previous library to iCloud Photos, because this will require a complete new upload of all your photos into iCloud Photos. Just downloading to your new, empty library will be faster.

If your Photos Library is in an unsupported location, you may want to drag it back to your Pictures folder or another supported location.