After upgrading to El Capitan Photos app crashes after 75% of conversion process. Neither rebuilding the library launching Photos with Alt-Command had success. The rebuild process stops at 5% saying the Library was not recognized. Same creating a new library from scratch and copying inside it the Masters and Resources folders. At the moment i cannot view my 150 Gigs of photos. See https://discussions.apple.com/thread/7252715?start=0&tstart=0
Create a new library, and import all of the other images from the Masters folder. Photos allows you to import images in sub folders, so dragging the masters folder should work.