Just got of the phone with tech support and this seems to happening quite a bit with Turbo Tax Premier 2020.
Hopefully you have saved what you have completed to this point and then Uninstall the program and reinstall it. You will need to reenter the LIC number and it should start right up.
April 2, 2021, I had to uninstall and re-install the software due to error code 1603. I encounter the same problem today and really do not want to save my file again to a folder, uninstall the deluxe software and re-install again. After reinstalling the software, I still had to manually add back the tax returns I completed. Too much work. Never had this problem with previous TurboTax products.