Lost photos from iPhoto library to Photos

I am missing a lot of my old photos from the old "iPhoto" app. I have pictures in "Photos", but am missing some very valuable pictures such as from my wedding and my child's baptism. I've backed up my computer many times and have had several OS changes since the iPhoto/Photo change. PLEASE help. I do not have an iPhoto library in my Pictures folder. I've already checked. :( Where did all the pictures go??