- Open your Game Library on Steam.
- Right-click on your game.
- Select Properties from the menu.
- In the Properties menu, select 'Local Files' from the left-hand panel.
- On the Local Files page, select 'Verify integrity of game files'
- Steam will then verify your files. ...
- Once Steam is finished, a box like this will appear.
If that's not enough, just reinstall the game.
Btw. save files are located in Steam\steamapps\common\Darkest Hour A HOI Game\Mods\Darkest Hour Full\scenarios\save games
or if you have mods, (for example famous Kaiserreich): Steam\steamapps\common\Darkest Hour A HOI Game\Mods\Kaiserreich-DH-1.9.3 Hotfix (12-21-2020)\scenarios\save games
So you can backup your saves if you want.