If you find your existing league won't load the game automatically saves a backup copy that you can use to reload from a previous successful save.
Find your league file in Documents\Wolverine Studios\DDSPF 2020\Leagues\YourLeagueName
The league file that loads is the .bin file. The backup is the .bak file.
Rename your YourLeagueName.bin file to YourLeagueName.old
Then rename the YourLeagueName.bak to YourLeagueName.bin
Your league should open now.
To prevent this in the future make sure you're not turning off the PC while saving. You can also check to see if your league folder is set up as an exception for any Firewall or Virus programs (Avast, etc), and you may want to remove your leagues folder from syncing with OneDrive if you have it set up that way.