How do I access iPhoto now?

Have you just upgraded to macOS 10.15 Catalina? Then iPhoto can no longer run on your Mac, because iPhoto needs some system frameworks with 32bit code, that are no longer available on macOS 10.15 Catalina. iPhoto has been replaced by Photos for Mac in 2015 and never been updated for newer system versions since then.

You can access the iPhoto Libraries holding your photos using the Photos.app.

• When you first launch Photos, it should automatically open the last use iPhoto Library and create a Photos Library from it. This will let you view and edit your photos.
• To open other iPhoto Libraries in Photos, quite Photos, if it is running, the launch it again while holding down the options key ⌥, until the library chooser dialog appears. Select the library you want to open.

Here is the Photos User Guide: Photos User Guide for Mac - Apple Support