If I started 2019 taxes can I still go back and file 2018 taxes?

You can purchase 2018 desktop software as a download at this page and install it on your computer:
https://turbotax.intuit.com/personal-taxes/past-years-products/

Minimum system requirements for Windows version:
https://ttlc.intuit.com/questions/1901571-minimum-system-requirements-for-turbotax-windows-software

Minimum system requirements for Mac version:
https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software

The 2018 return cannot be efiled.  It will need to be printed, signed, dated, and mailed.  It's a good idea to mail by a method that has tracking, such as certified mail, priority mail, etc.

If you have both Federal and state returns, you need to mail each one separately to its respective tax agency. 

If you are due a Federal refund, it takes about 6 weeks for the IRS to process a mailed return and deliver the refund.

Or did you already start a 2018 return online prior to the 2018 online shutdown, and want to finish that one without starting over?  If so, you can download the *.tax2018 data file from your account and finish it in the 2018 desktop software.    If you already paid for your product previously, TurboTax Support should furnish you the software.

OR another option if you have a simple return is to download the 2018 forms and instructions for free from the IRS website and fill them out manually.

https://www.irs.gov/forms-pubs/prior-year