If you have a version on disk, either from the computer's original disks or a backup, install or restore that.
If you don't and the computer shipped with Mac OS X 10.6.8 or earlier(not including Mac OS X Server), phone AppleCare and order replacement disks.
If you don't and the computer shipped with Mac OS X 10.7 or newer, go to the Mac App Store main page, click on Support under Quick Links, and contact Apple.
If the computer shipped with Mac OS X Server 10.6.8 or earlier and you didn't already buy iPhoto, buy an iLife 11 DVD from a source such as Amazon or eBay.
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