When you use a FREE anything there are always limitations ... and not having access to your returns AFTER the end of season is one of them ... your choices are to pay for the PLUS upgrade to keep access OR save a PDF of the return while you still have access to it. The option to upgrade is offered and explained no less than 3 times as you use the program and you had to decline the update each time ... do not be mad at TT for the decisions you made. Now upgrading to PLUS is not an option as the online system is changed to the new tax year... this blackout period happens each and every year so all you can do is wait for the system to come back online OR get a free transcript from the IRS as mentioned in the other answer.
Note: Every year, before mid-October, print a copy of your current year tax return and save it as a tax file and pdf, too. That way you will have it when you need it for school, a loan, next year’s tax return, etc. without going through a lot of aggravation (and expense) to get it.
https://ttlc.intuit.com/questions/1899645-how-do-i-save-my-2018-turbotax-online-return-as-a-tax-data...
https://ttlc.intuit.com/questions/1912544-how-do-i-save-my-2018-turbotax-online-return-as-a-pdf
