See this TurboTax support FAQ for when access to prior year online tax returns expire - https://ttlc.intuit.com/community/prior-year-return/help/when-does-access-to-my-prior-year-turbotax-...
You can also get free transcripts of your federal return from the IRS - http://www.irs.gov/Individuals/Get-Transcript
In TurboTax Free Edition, you can purchase PLUS to unlock your prior-year .tax file.
The exact instructions will vary depending on the status of your 2018 TurboTax Online return (select one):
