The TurboTax online returns for tax year 2015 have shut down to make way for the 2016 online editions.
If you paid for one of the 2015 online editions of Plus, Deluxe, Premier or Home and Business, then you will need to contact TurboTax support and speak with an agent to get a 2015 desktop installation file pushed to your download account.
If you did not pay for one of the 2015 online editions or only used the Free edition, then see this TurboTax support FAQ for completing a prior year 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...
Download the tax data file from your 2015 online Timeline and then open the file with the 2015 desktop program to complete the tax return. A 2015 tax return can only be printed and mailed, it can no longer be e-filed. See screenshot
See this TurboTax support FAQ for a contact link and hours of operation - https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
To contact TurboTax customer service/support use their contact website during business hours.
Submit your question/problem during business hours and then use the option for Call (a phone number will be provided for Call) to speak directly with a TurboTax agent -
https://support.turbotax.intuit.com/contact/
Note: The Call option will not be available when TurboTax Support is closed.
