first thing to try is a complete power re-cycle of your network.
Power off your modem, your router (if you have one), your PCs, laptops, xboxes. Power off everything on your network.
Note: some modems have a battery back up and can't be powered off. Use the "Reset" button on the modem in this case
Let the whole system set unpowered for 30 minutes or more.
Then power them back on in this order.
1. Power on the modem (Wait a few minutes for the lights on the modem to settle down)
2. power on the router (Wait a few minutes here too)
3. Power on your PC (Check to see that you have internet connectivity)
4. Power on your xbox (Hopefully all works well at this point)
5. After your xbox is working properly and with your xbox still powered on, turn on the other devices on your network.