If iPhoto will Not Open a Library Again, That you Migrated to Photos

If you open an iPhoto Library in Photos, the Photos for Mac application will change the filename extension of the library from ".photolibrary" to ".migratedphotolibrary". iPhoto will not open this library, and neither Photos.

To open the library again in iPhoto, edit the filename extension back to ".photolibrary". You can do that in the Info panel for the library.

To open this library in Photos to repeat the migration, see this support document.

If Photos won't open a library that you already migrated - Apple Support