I had this problem on a number of occasions with my '99 STS. I found that after shutting down the vehicle and doing a restart rebooted the vehicle computers and it the problem was resolved.
However, we have not as yet experienced the same with my wife's NM XLR.
I can only assume that this has been a periotic event and one that will not occur when you take your XLR to the dealer. If this is the case I would still recommend that you report this to your dealer and let them give your vehicle a physical. It is important to report events, even if the dealer can not evidence them, should there be a massive falure.