Portal isn't working!

When i start up portal it goes throught the valve logo and the valve music plays but once its done with that i get kicked out. It doesn't even give me an error or message. Please help :'(

Did you download it from Steam?

I had exactly this problem (if you got it free on Steam). But now it works. I let it update, maybe they released an update that fixed it. Or is yours already up to date?