Modern Warfare 2 Multiplayer: "Fatal error: steam must be running to play this game"?

So i bought MW2 for pc on steam about 2 months ago. It worked fine. Then after 2 months i try to play MW2 multiplayer again (btw i noticed that steam had changed language from finnish to english) and that error pops up. Then it just closes. MW2 Singleplayer works fine. My game is legit. What could it be?