What if you copy your Documents/Paradox Interactive/CK3/last_save.ck3 game into the /save game/ folder, doe sthat one load okay then?
Look in your Documents/Paradox Interactive/CK3/ folder. If you see last_save.ck3 there AND you have not since played any other campaign, you can copy that file into the /save game/ folder there and see if it's the one you need.
Good luck!