Xbox Live Problem

Purchased Xbox Live During Summer 05 And Was Working Perfectly With Wireless Router. Purchased Tom Clancy's Splinter Cell Chaos Theory And Used It And Since Then Every Time I Turn On Xbox It Says It Needs Service. I Turn Off And Back On And Says Xbox Live Not Working But Am Able To Play Regular Xbox Games. How Can I Get Back On To Xbox Live?
Please Advise Asap!
Thanks!