It used to be a common problem, losing the XM bands. I think there were basically 2 issues, a timing issue within the HU or a connection fault. Easiest thing to try first is disconnect and reconnect the cable at the XM box over the left rear wheelwell.
Read through here for ideas but look at post 9 for sure:
Yes, pull the carpet back from the top, and if the box is still installed, will be staring at you on top of the drivers rear wheel well in a coupe, or in a convert, its under the waterfall cover between the seats.