What you have is an iPhoto library that has already been migrated to Photos. It must have been accidentally deleted or moved to a different location. What do you see when you launch Photos with the Option key held down? Do you see any libraries to open?
First I suggest you try to restore the Photos library from the backup made just prior to you losing the library. Go to the Pictures folder, enter Time Machine and go back in time till you see a library with this extension: .photoslibrary. Restore that to the Pictures folder and open.
Otherwise you can change the extension on the one library you have to .iphotolibrary and open it with Photos. A new Photos library will be created and the current library will again have the .migratedphotolibrary extension. Do not delete the migrated library until you're sure the new Photos library is complete and working properly.
