Did you use TT online (web) or TT Desktop (CD/Download)?  If the latter, they will be only on the PC you used to prepare them.  If instead you used TT online, be sure you are logging in with the correct User ID and password for those years.

I need a copy of my taxes from 2016. Is it possible to have them downloaded or mailed to me

I filed  2018  online but 2019 was filed with a download from turbo tax purchased from Costco. My user id from those years do not seem to work now. I changed my password about 6 times today. I am very frustrqted with your service. Your support staff was not helpful

You can use this link here to recover your account. There is a small blue link at the bottom that says Try something else and if you click on that you can locate your account by using your SSN, name, and birthdate. If you still can't access your account, you can contact TurboTax Support for assistance by following this link here.