When you use online TurboTax software you get one return per fee.
Each return needs its own account and user ID. If you use the same account and user ID for a second return, the second one overwrites the first return and it is lost forever.
If you are using TurboTax Desktop you can click the blue "Start a New Return" button on the main screen when you first open the program (see screenshot). If you're using TurboTax Online you'll need to create a new account.