Do you mean the Steam client or does the ck2 start window show at least ?
First of all, uncheck any mods if possible.
Second, check the version number and if the checksum fits.
If you can't even load the ck2 window, try running ck2 without steam and directly from the ck2game.exe in the folder (not! from the shortcut).
If it runs without Steam, try updating your Steam app.
If nothing works, i can only suggest deleting the ck2 folder in documents and if that doesn't help, then a clean reinstallation of the game.
- move any valued save games elsewhere
- "delete local content" in Steam-CK2
- manually delete all files and folders left under Steam/CK2 AND Documents/Paradox/CK2
- re-install game in Steam, run a Steam Verify when done.