You would file a 2020 tax return. If your check was sent to a closed bank account, you will probably receive a check or debit card through the mail. The IRS is not accepting tax returns until Feb 12, so there is no need to file a 2020 tax return before then. Chances are high that you will receive the payment via mail before then. But if you don't, you can claim the Recovery Rebate on your 2020 tax return.