One thing you might try is to open Turbotax 2020 and open your 2020 tax return. Then save it under a new name in a location you can easily find (eg, desktop) using "save as". Then close the TT 2020 version. Open your 2021 version and look for the file where you saved it. I assume you have downloaded versions of TT on your PC. If not, then not sure what else to try.
You can also run a search for files ".tax2020" on your harddrive. It is possible that the file could be corrupted. That sometimes happens on harddrives. I always save another copy of my tax files in a different folder as well as on a backup drive. Good luck.
