I filed a 2015 federal return but did not file a state. How do I file my state return?

 Online preparation and e-filing for 2015is permanently closed.  You have to download 2015 software to a full PC or Mac (not to a mobile device) and then print, sign and mail your return.  If you already paid for your TurboTax software, you can contact customer support to push a download of the software to you.

To call TurboTax customer support

https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number

They are available from 5 a.m. to 5 p.m. Pacific time.

To finish a prior year return started in TT online:https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS and/or your state received the return.

Another possibility:  Get your 2015 federal return in front of you and obtain the tax forms from your state's tax web site and prepare the state return by hand, then mail it in.