Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
The problem is, Apple has replaced iPhoto with new application called Photos app. You can open Photos app from Applications fold or from dock. 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. If Photos doesn't open your photos automatically, then hold down Option key key on your keyboard and open Photos. Select your library and Choose Library. This will open the photos in Photos app.
(You can open Applications folder from the left panel in Finder or from Go menu in the top bar)
Please let me now if you have any difficulty, so I will continue to help you.