[2021] Corrupted Save File?

Not sure why that happened to yu, but once a computer file is corrupted there's no way back - sorry!

However with this game there's an emergency copy Documents/Paradox Interactive/CK3/last_save.ck3 - IF you have not started or played some other campaign since! Does that help you?  

Move that save file to Documents/Paradox Interactive/CK3/save games/ folder, and it will show in the game to be loaded.