Have you been on XBL since you modded it? I suspect that you may be banned, before you go onto XBL, your Xbox's memory section is scanned for cheats and homebrews if either is detected you will get banned. What a lot of people do not know is that anytime you use an App, such as XBMC, DVD2Xbox, or the Region Free DVD player, it leaves a file in your memory section. So prior to going on XBL you must always go to the MS dash memory section and delete these files. For if you do not they will know you have a modded Xbox and ban you.
Well, check your settings and make sure their all the way you had them because my friend had the same prob and it was that the DNS was changed to manuel and he changed it back then it worked fine
