It could be a few things. Let's work through them.
Option 1) It's always possible you're in the wrong account. It happens quite a bit that users (myself included) create more than one username and log in to the wrong account.
If you're not seeing ANYTHING - as in the page reads "Let's start your 2016 return" when you log in, then it's a good indicator you're in the wrong account.
If that's the case, you can call us (log in, click the "Help" or "Expert Help" in the top right and search "Contact") and we'll guide you to the correct account.
Option 2) TurboTax Online becomes inaccessible after the extension deadline each year (Oct 16th, this year). We take it down to make room for the new year's software to be rolled out in December.
If you've logged back in after that date and had been working on your return, you now need to download your ".tax" file and the software package. Instructions are included on the home page.
The ".tax" file has all of your information in it and it can only be read by the TurboTax Desktop software. So, you'll need both to keep working on your return.
Those are the two most common cases you'll find for not being able to locate the return you'd been working on.
If you're on TurboTax Desktop (where you actually downloaded and installed the software onto your computer) and you can't find your files, try searching your computer for files ending in".tax". Usually the software saves it as "UsernameTax.tax2016".