You have "Help" in the title, yet you don't even post in the help section. Damn Choob.
You have edited the textures or deleted one.
Go to the Games tab.
Right Click Modern Warfare 2 - Multiplayer.
Click Properties and go to the local files tab.
click "Verify integrity of game cache..."
It will check you MW2 files and automaticaly redownload the non-messed up files.
If that doesn't work just post here.