If you completed the 2015 tax return using the online product, you should be able to find it by using this link:
If the return was not completed, it is too late to complete it now because TurboTax no longer supports tax year 2015. It is also too late to get a refund on a 2015 tax return. It had to be filed within 3 years of the original filing date to be eligible for a refund.
You can check to see if the unfinished return is still in your online account if you know how to access the account. It is too late to receive a refund for a 2015 tax return, but if you owe tax due, the IRS still wants to be paid. You cannot e-file a 2015 return. In order to file it you have to print, sign and mail it.
Many people have multiple TT accounts and forget how to access them. Log out of the account you are in now.
Or did you use the desktop version of TurboTax? If so, the files are on your own hard drive or any backup device you used like a flash drive.