Was your 2017 return prepared with Online TurboTax? If so, log in to your account and look in the section "Your Tax Returns & Documents." Choose tax year 2017, and look for a link to download/print the PDF. Do you find evidence of your return there OK, but when you try to download, does it ask you to pay? If it was prepared in Free Edition last year, access will be locked. If that's your situation, we can tell you how to pay to unlock the 2017 return. The method to unlock it depends on what all you have done with your 2018 return.
If you find no evidence at all of your 2017 online return in that account, we can tell you how to look for multiple accounts. A return prepared in desktop software (CD/download) will not be in an online account.
Have you filed a 2018 return in Online TurboTax, or is one in preparation? Or did you file elsewhere?
Was the 2018 return prepared in Free Edition?
Is the 2018 return in the SAME account as the 2017 online return?
We can tell you how to unlock a return once you answer all the questions above.
There's also a free option if a return transcript will suit your purpose. You may be able to immediately download one at the IRS website if you qualify to open an IRS account there. Otherwise, to order a transcript to come by postal mail takes 5-10 calendar days.