You may have started a new return at some point, and now you may not be able to get to your original return – which you have paid for and completed.
Try clicking on the "My Accounts and returns" in the left-hand vertical nav. And then "View all tax returns".
You will see a screen that shows you if you have started multiple returns using the same account. See example screenshot. The paid and completed return you're looking for may be there. Hope this helps.