I had a similar issue, in the end it turned out to be a corrupted save file. Chances are your most recent savegame didn't save properly and that's the cause of it not starting. Deleting it soled the problem just fine.

Every time I go to start CK2, steam will say its running next to the game name and then it just stops. Every once in a while it gives me an error message saying the memory could not be written. I have re-installed the game multiple times, but it doesnt fix it. I am out of things to try that I know how to do because I am not computer savy at all. Has anyone had a similar problem or suggestions for things I can do? My other games on steam start fine. CK2 is the only one not working.

CK2 wont start