If you used TurboTax Online last year, sign in to TurboTax Online for the tax year 2016 here: https://ww1.2016.turbotaxonline.intuit.ca/.  If you used TurboTax CD/Download and you saved it on your computer you can simply go to the Start button on the bottom left of your screen and click on Start and you will see a small search bar - type in .tt16 and you will find it there.  If you did not save it, unfortunately, there is not a way to access it.

When I try to access this website, I get a message telling me the site isn't secure

TurboTax Online for tax year 2016 has now been retired to make way for the upcoming release of tax year 2019. Please refer to your saved PDF copy of your return or sign in to your CRA My Account to see details of the filed return