You could try recovering your gamertag on a friends xbox and try that or try a different game disc. I had a problem the other week where 3 times in a row it wouldn't let me online on gears 3 only so I put my brothers disc in and it worked. Took it out and put mine back in and I was back online. Strange thing but that's how I fixed it.
If the above fail maybe your gamertag is banned from playing it!