If you are using the TurboTax online web-based editions those are only good for one tax return per account and User ID. To start another tax return using the online editions you will need to create a new account with a new User ID and a new email address.
Go to this website and click on Sign up to create a new account - https://turbotax.intuit.com/personal-taxes/online/file-your-own-taxes/
