TT Desktop is program you use by buying a CD, or downloading and installing the software on your hard drive. Any tax returns you prepare reside only on your hard drive.
Conversely, TT Online is web based, and returns are stored on TT's servers.
You have to sign onto your 2015 online account using the exact same user ID you used to create the account. Use this TurboTax account recovery website to get a list of all the user ID's for an email address. Run the tool against any email addresses you may have used - https://myturbotax.intuit.com/account-recovery
If none of the user ID's received will access your 2015 account, then use the option shown in blue on the account recovery website, "I don't know my email address or user ID"
If you have filed your 2015 tax returns and they have been accepted, when you sign onto your online account and are on the Welcome Home screen, click on Taxes at the top of the screen. When you are on your 2015 tax timeline, under the section for SOME THINGS YOU CAN DO, click on either option for Download/print return (PDF) or Download all forms and worksheets
In the section Looking for prior year returns? Click on Load my tax timeline to access prior year tax returns.
Once the PDF has been downloaded to your computer, you can view or print the return with any PDF compatible viewer, such as Adobe Reader.
If you used the desktop CD/Download editions installed on your computer, the only copy of your tax data file and any PDF's will be on the computer where the return was created. TurboTax does not store online any returns completed using the desktop editions.