Photos App Won't Open

Oops. You could do a search on your HD for .photoslibrary as this is the suffix the library has.

Do you can iCloud Photos enabled? If so you can create an empty library and then make this the system library and it will download all of your photos from iCloud.

Otherwise, do you have any backups, Time Machine?

Have you moved or re-named your library?

You can either double click the library directly to open it or hold down Alt when you open the Photos.app to select another library.