Q: lost aperture when upgraded OS

If you need a feature which Aperture has but Photos doesn't, click here and follow the instructions.

(137211)

Which version of Aperture do you have?  The link Niel posted should allow you to update any AppStore version of Aperture 3.  If your Aperture 3 is not from the App Store, contact Apple Support and request a redeem code. You will need a proof of purchase and be insistent. (Re: aperture updates)

If you cannot update Aperture because your version is Aperture 2 or older, you can always open your Aperture library in the new Photos for Mac application.  Photos will transfer most of your metadata and edits losslessly. See this link: How Photos handles content and metadata from iPhoto and Aperture - Apple Support