There are a few potential reasons why your Call of Duty Modern Earfare isn't working. The first thing you should do is check to ensure that your system meets the minimum requirements for the game. You can find this information on the game's product page or online. If your system meets the minimum requirements, you may want to try updating your graphics drivers and/or DirectX. You can find instructions on how to do this online as well. If updating your graphics drivers and/or DirectX doesn't solve the problem, you may want to try reinstalling the game. Lastly, if none of these solutions work, you may want to contact Activision customer support for further assistance.