You should be able to log on to your account and select the 2017 tax year. If you filed more that one state return though TurboTax will ask you to pay again for the extra state forms. This has been an issue for me that is currently being resolved.
It sounds like you did not start out using the same account and user ID that you used for 2017, or you would have already had access and would have been able to transfer the 2017 information in to 2018 at the beginning. Now you want to get to your 2017 return? You say your 2018 return is not yet complete--you could consider starting over using the same account and user ID so that you could transfer 2017 in to 2018 That way you would have the 2017 AGI you are going to need in order to e-file 2018, and your personal information and some of your income information would also transfer in.
.https://ttlc.intuit.com/questions/1901535-forgot-your-turbotax-online-user-id-or-password
https://ttlc.intuit.com/questions/3404575-how-do-i-access-my-prior-year-2017-return