Online TurboTax is currently in the November limbo period, which occurs each year after it shuts down after filing season. Online TurboTax for 2019 opens the first week of December, and you can upgrade a 2019 return to PLUS at that time, which will unlock all prior returns.
Or you can try this as a possible workaround:
You may need to wait to try the steps during hours when Customer Support is open. That interface may not be fully functional when they are closed. But here is a possible workaround to use when they are open. Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Mon-Fri. They are closed on weekends this time of year.
Log in, and at the main Tax Home page immediately go to the top right corner and click HELP. Scroll way down beyond the suggested links and click on CONTACT US.
You may have to enter a question.
Choose to phone TurboTax Support, or it may say schedule a callback.
In any case, it may then ask you to pay for PLUS (19.99) to have phone support.
Once paid, then past returns should be unlocked.
It's not clear if during this November limbo period that method pays for 2018 PLUS or 2019 PLUS, but in any case, the net effect is unlocking prior returns, plus it gives you phone support privilege. If it pays for 2019 PLUS, then you have an extra year of access. You should be saving a copy of your return and/or PDF each year anyway (in a protected and secured safe place) so you don't have to worry about access problems.
If you can't use that method, another option is to download a free 2017 tax return transcript from the IRS if you qualify to open an IRS account. A transcript by postal mail takes 5-10 days.
Also, if you need a prior return urgently and can't unlock it by that workaround above during business hours, then you can contact TurboTax Support via messaging at Twitter or Facebook, tell them your situation and that you are a victim of the limbo period, and see if they will unlock the returns for you.