Instructions
Restart "Call of Duty 4: Modern Warfare." Sometimes the files for entering a multi-player match are not properly booted when the game starts, so restarting your computer can retry booting the process for all of the game's files.
Patch your version of the game. Infinity Ward, the game's developer, has released several patches for the game that include a bevy of bug fixes and connection issues. Additionally, you will need the most current version of a patch to play in a majority of the multi-player matches. These patches can be found on a variety of file-hosting and fan sites of the game for free, such as FileFront, Strategy Informer and CoD4 Central.
Reinstall the game. Files can become corrupted for a variety of reasons and sometimes the only way to fix them is to reinstall the game. If a file that is crucial to connecting to the game's multi-player component has been compromised, you can restore the file back to its original state by completely reinstalling the game.