If you started the 2018 tax return using the TurboTax online editions and did not finish the tax return then see this TurboTax support FAQ for completing the return - https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-started-in-turbotax-online/00/26322
If you paid for one of the TurboTax online editions of either Deluxe, Premier or Self-Employed then contact TurboTax support and they will push a 2018 installation file to your download account at no charge.
Support is open from 5am to 5pm Pacific time, Monday thru Friday
Use this website to contact TurboTax support during business hours - https://support.turbotax.intuit.com/contact/
Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax