If you did not pay for the one of the 2016 online editions of either Plus, Deluxe, Premier or Self-Employed you will need to start a 2017 tax return using the online Free editon and create the account with the exact same user ID you used to create the prior year online account. After starting the tax return with the Free edition you will need to upgrade to the Plus edition and pay for the Plus edition so you can access the prior year online tax returns.
See this TurboTax support FAQ for accessing prior year tax returns - https://ttlc.intuit.com/questions/3404575-how-do-i-access-my-prior-year-2016-return
To access your prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)
You can also get free transcripts of your federal return from the IRS - http://www.irs.gov/Individuals/Get-Transcript