Solution 1 : Sometimes waiting helps. Like 10+ minutes and suddenly game continues.
Solution 2 : You can try :
1. First open your Steam games window-screen.
2. Click on Modern Warfare 2 (DO NOT CLICK MODERN WARFARE 2 MULTIPLAYER)
3. Then, at the bottom right corner of your screen, click on properties.
4. Go onto the tab Local Files.
5. Click on Verify Integrity of Game Cache. Let it run and download any missing components.
IF this does not fix your problem repeat this for Modern Warfare 2- Multiplayer.
Hope that helped :)