Did you try verifying local files?
Try starting the game directly, rather than through Steam (exit the Steam client first, to isolate whether the problem is with the game or Steam). There are 2 executables in the Divinity 2 install folder, Divinity2.exe (normal DKS) and Divinity2-debug.exe (with the Developer Mode).
Have you installed / changed / updated anything recently?
Are you shutting down all non-essential programs (especially anti-virus) before trying to start the game?
