How can I get back my iPhoto

iPhoto is not compatible with macOS 10.15 Catalina.

You can access your photos in your iPhoto Libraries by opening the iPhoto Libraries in Photos for Mac, the successor to iPhoto. See: Updating from iPhoto to Photos for macOS – Apple Support

When you launch Photos for Mac it should automatically open the last iPhoto Library you have used. If not, hold down the options key ⌥ while launching Photos and select the library from the library chooser dialog.

How did you lose it? iPhoto hasn't been sold or developed for 5 years now.