If you used TurboTax Free to complete your 2018 tax return and did not opt to add the "PLUS"- then your return will not be saved.
Tax return access services are now included, for the first time, in all free and paid products beginning in the 2019 tax year, regardless of whether or not you have added any PLUS features onto your account.
To find your filed online returns click here.
If you are asking if your 2018 return was NetFiled- you would have received a Notice of Assessment by mail a month or so after you had filed your return. If CRA doesn't have your return from 2018- then you will need to complete a new 2018 return.
Click here to check the status of your return.