To access prior year online tax returns you must have used and paid for one of the 2016 online editions of either Plus, Deluxe, Premier or Self-Employed. You must sign onto the the TurboTax online account using the exact same User ID you used to create the account. Access to the prior year online accounts is good through October 2018.
See this TurboTax support FAQ for when access to the online accounts expire - https://ttlc.intuit.com/questions/3355524-when-does-access-to-my-prior-year-turbotax-online-returns-...
If you did not use and pay for one of the 2016 edtions, then you will need to start a 2017 online tax return with the Free edition and then upgrade to the Plus edition. You would then need to pay for the Plus edition upgrade so you will have access to the prior year online tax returns through October 2019.
See this TurboTax support FAQ for upgrading to Plus - https://ttlc.intuit.com/questions/2586262-how-do-i-purchase-turbotax-plus-so-i-can-get-my-tax-file
You can also get free transcripts of your federal return from the IRS - http://www.irs.gov/Individuals/Get-Transcript