What? That happen to me today too, there are no infos or any kind solution for this problem. I guess I will have to stop by Xf office tomorrow.
If it's not a AC outlet issue, then it's time to take the modem / gateway to the local Comcast office for a swap, or call for service and have a tech bring you one.
Make sure the power line is fully inserted into the modem. If your modem uses a separate power adapter, it may be bad.