If you used the desktop software, they are saved and stored on your computer; they are not stored in the cloud. If you used TurboTax Online, you can access them by signing into your account and scrolling down to the bottom of the page and expanding the section titled Your tax returns & documents.
Make sure you are signing in to the same account you used to file the returns you are looking for. How many TurboTax Accounts do I have?