I can't think of a reason that 2019 data that transferred into a 2020 return would "disappear" unless you have multiple accounts and started a second 2020 return in another account.
I can tell you how to "clear and start over," but first be sure you are in the correct account and don't have more than one account. Here's one way to tell:
In the current account you are using, log in (if not already in), and at the Tax Home scroll down to the section "Your Tax Returns & Documents." Expand that section and choose tax year 2019. Is the 2019 return there? If so, view the 2019 PDF to be sure that the PDF has the complete final 2019 return that was filed last year.
If the above shows you are in the account with the filed 2019 return, then I'll tell you how you can clear the 2020 return and start over. It should transfer the 2019 data in when you restart.
