Lost Photos Library

Not sure if you are missing the Photos Library or the original iPhotos Library.

The original iPhoto library should be in the same location as it was when you migrated it to Photos. The name will change to something like migrated.iphotolibrary (can't remember exactly how it was named). When you open that library with iPhoto, it should revert back to the original name.

You can't merge libraries directly from Photos. There may be a 3rd party app like iPhoto Library Manager or others. You can google that question for answers. I'm not familiar with them.

I did use iPhoto Library Manager to merge previous iPhoto libraries in the past. Has Apple provided a merge tool in the latest version of Photos?

As an add-on to Glen's post: There is a successor to iPhoto Library Manager from the same manufacturer - Power Photos. It can merge Photos Libraries, see this page: Fat Cat Software – PowerPhotos

The manual is here: https://www.fatcatsoftware.com/powerphotos/Help/table%20of%20contents.html

Photos itself can only merge libraries by uploading libraries to iCloud Photo Library. If you sync two libraries with iCloud, one after another, they will merge in iCloud and download to your current library as a merged library. But it will require a paid storage plan to accommodate a large library in iCloud.

When you opened your iPhoto Library in Photos, the iPhoto Library changed the filename extension to ".migratedphotolibrary". Change this filename extension back to ".photolibrary" and iPhoto will see it again.

You are welcome, Robb! 🙂