I'm sure that you know your GamerTag correct?
The easiest thing to do is to call 1-800-4MY-XBOX to reach Microsoft's technical and account support services, or you can reach Microsoft online (http://support.microsoft.com/common/international.aspx?RDPATH=dm;en-us;select&target=assistance) but I prefer talking to a live person.
Explain your (common) situation and they will be able to help reestablish your XBL access!
Hope this helps, and if it resolves your problem, please remember to click accept as I do not get compensated for my efforts until you do. Bonuses and positive feedback are also greatly appreciated.