You will have to log on using the account information you filed last years taxes with.
Note:
Access to prior year returns is not a feature of the Federal Free Edition. You will need to upgrade your version of TurboTax, purchase a "Plus" package or request a transcript of your prior years return from the IRS (Link at the bottom of this answer). The price you pay for the upgrade comes with more features than just access to your prior years returns. the features of the various TurboTax products are explained on the TurboTax website listed below: