Retrieve lost iPhoto after replaced hard drive
If you backed up all your files, then your library has been backed up and all you are missing is the iPhoto app. The iPhoto app has been replaced by Photos.app in Yosemite, so you can re-download iPhoto or Photos from the Mac App Store and your library will be automatically detected.

If you didn't back up all your files, then you need to rely on the last one thousand photos accessible in Photo Stream or Shared Photo Streams.