why didn't all of my photos transfer to the new OS X Yosemite from my old OS X version?

I just had a new operating system installed (OS X Yosemite 10.10.1. When I tried to open iPhoto to view my photos from the old system, a message indicated that I needed the newer iPhoto installed before I could view the photos I had stored on my old version. After installation, many of the photos from the old iPhoto did not transfer. How can I retrieve the old photos or are they lost forever?