Please edit the file Documents/paradox/ck2/settings.txt to say:
fullScreen=no
borderless=yes
THEN go here:
https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads
And run the x86 and x64 installers there.
IF that doesn't help, has the game ever run OK on this computer? Or never?
IF NEVER:
Install ALL these Microsoft modules:
https://www.microsoft.com/en-us/download/details.aspx?id=26999
https://www.microsoft.com/en-us/download/details.aspx?id=52685
https://www.microsoft.com/en-us/download/details.aspx?id=8109
(run dxsetup.exe after extracting this download to its own folder anywhere on your computer)
Add ck2game.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
Also if you have installed RivaTuner, RadeonPro, or a gaming VPN app called Evolve, or the MSI Gaming App, or Duet, disable it as it clashes with this game.
IF IT USED TO WORK:
Please do a full **clean** re-install:
- move any valued save games elsewhere
- "uninstall" in Steam-CK2
- manually delete both the Steam/SteamApps/common/CK2 AND Documents/Paradox Interactive/CK2 folders
- re-install game in Steam, run a Steam Verify when done.
- add ck2game.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test
In particular make sure there are no files or folders left under either the Steam CK2 or Documents CK2 locations, before installing again.