Separate download of Photos App for Yosemite
If you use Time Machine, just restore the app (and its data, if applicable) from an older backup. If not, reinstalling OS X is probably the best idea, as Photos is not available on the Mac App Store and I definitely wouldn't try to download it elsewhere (so don't just type "Mac Photos.app installer" in Bing or Yahoo or Dutch and download whatever comes up). You could copy the app over from a friend or family member, though, if you wish.