Many people noted this problem when they installed Photos app on their mac. In some cases, iPhoto is disappeared from the Dock or it has been deleted from the "Applications" folder or if it is out of date and could not be upgraded to a Yosemite compatible version.
In these cases, you need to re-download an iPhoto from the Mac App Store. To do so, launch the Mac App Store and click the "Purchases" tab. If iPhoto does not appear as an option, then scroll to find it. If you see "Install" label then click it. If you see "Open" label, then click "Open" and if it launches successfully, then it is still available to use. If iPhoto refuses to launch, then find it in the "Applications" folder and delete it and empty the trash. Again go to the Mac App Store's Purchases tab and click "Install" button.
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