Same happened to me but I found a link that helped me re-install it. (couldn't find it now for reference)
Try looking for it in the Mac App Store for download. If it's not there, try creating a new user account on your Mac. Log into the Mac App store using your iCloud info, then look for it again in your purchases. It should appear under the purchases list. From here you can download and install it again and it should be available in your main user account. At this point, you can delete the new user account. iPhoto should be available now.
This trick worked for me. I hope it works for you too.
It does, but neither of those installs will erase iPhoto if you just do an upgrade. It sounds like maybe the OP did a clean install by erasing the drive then installing the OS.