You should be able to access your prior-years' returns up to 7 years past. The only thing I can think of is that maybe you used different login info (username, password). In order to access your 2015 return, you'll need to use the EXACT account that you used in 2015. If you need to recover the login info, you can use the instructions I'm linking below.
Forgot your TurboTax online user ID or password?
If you still have difficulty, you can contact TurboTax Customer Support for assistance.
How do I contact TurboTax?
You may also be able to find the address you filed from by getting a transcript from the IRS. It's online and it's free.
Get Transcript | Internal Revenue Service (irs.gov)
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
