So I tried what someone suggested: holding option and command down while opening Photos.
A popup then opened but had nothing about a Library of any kind.
There are two key combinations to be used with Photos:
- Hold down the options key ⌥ when launching Photos. keep holding down ⌥ until the library chooser dialog appears. This will let you create a new library or select one of the existing libraries.
- Or hold down option-command ⌥⌘ while double clicking one of your libraries. This will run a repair on the selected library.
You may have been looking for the first option ⌥ to open the Library Chooser. The second option - repairing a library - should only be used, if the library has become corrupted. It is a very expensive operation, It may take several hours or even days to run, depending on the size of the library. For an iCloud Library it will result in a new upload of the complete library into iCloud. And it should never be used without making a backup of the library before you try it. The problem is, if a library is already damaged, trying to repair it may damage it further, so you need a backup.
Where is your Photos Library? is it in your Pictures folder or on an external volume? If it is on an external volume, what is the file system format? And which version of MacOS X do you have installed? Are you using iCloud Photos? And have you already tried to restart your Mac?