An extension for tax year 2017 would be valid until October 2018, so you must mean an extension for tax year 2016 which you filed during calendar year 2017 on or before April 18, 2017. The deadline for filing a 2016 tax return with extension was October 16, 2017. Unless you lived in one of the states affected by a Federally declared disaster, then the deadline to file a 2016 tax return with extension is January 31, 2018.
See this TurboTax support FAQ for completing a prior year tax return started with the online editions - https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb...
If you did not start a tax return using TurboTax then go to this website to purchase a download of the 2016 desktop editions - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2016 tax return can only be printed and mailed, it cannot be e-filed.
