It happens---sometimes users think they e-filed and do not notice the return was rejected so they do not follow up and fix it by making corrections or mailing it instead. Or they pay the TurboTax fee but do not go all the way through and click the big orange button that says "Transmit my return now." If that is what happened--you can still file it but the only way to file it now is by mail. E-filing is closed for 2018.
https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.