There's at least two explanations for what you can't access Marketplace.
1. Marketplace is current inaccessible at the time you're trying to access it. Happens to me roughly once every couple of weeks.
2. There's an issue with your Live ID. This happened to me months ago when I tried to download an app. It gave me an error message that, as best I can remember, simply said there was an issue with my Live ID or password. I signed on to Live on my laptop and it told me my password had expired. I've gotta believe that this would've generated some sort of error code on MS' side. This error should be relayed to the user.