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The problem is, Apple has replaced iPhoto with new application called Photos app from OS X 10.10.3 onwards. You can open Photos app from Applications folder. Photos app does support your old iPhoto library. So you will be able to open Photos and see all the photos that were in iPhoto.
Open Applications folder and you will find the Photos icon (a flower like icon). Open it and it will import your iPhoto Library automatically.
If it doesn't, then close Photos. Hold down Option key on your keyboard and open Photo. Select Choose Library, then from the list of library select iPhoto Library and choose it. This will migrate the iPhoto Library to Photos.
Please let me now if you have any difficulty, so I will continue to help you.