If you have an active subscription, don't have any enforcement action affecting your connection to the game servers and your privacy settings all enable/allow online multiplayer, then be sure that you're signed in to the correct account/profile.
Be sure your PC meets the minimum system requirements for Call of Duty: Modern Warfare II. If you're experiencing graphical issues in Call of Duty: Modern Warfare II, be sure to install the applicable video card drivers listed below. Nvidia: The recommended driver version for Call of Duty: Modern Warfare II is 526.86.