I lost my iPhoto when I upgraded, how do I get it back?

they stopped iphotos and introduced an app just called photos as it's replacement

have you tried to start that?

What version of the OS do you have? iPhoto v9.6.1 runs on High Sierra - and that's the forum where you posted.

If you're open Cataloina (OS10.15) then

iPhoto was discontinued 5 years ago. It is no longer developed or sold. It was replaced by the more powerful (and free) Photos.app that comes with the OS. You can migrate to that app very simply:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.

If you prefer not to use Photos there are several 3rd party apps that you can use. Which to choose depends on the kind of photographer you are and your budget.