Reset Xbox Settings
A setting may have been changed that's preventing access to Xbox Live if you were able to connect successfully in the past. The Xbox 360 console includes an option to automatically revert all network connection settings to their default values in the event of a problem. Navigate to the "System Settings" tab in the "My Xbox" dashboard and open the "System Settings" option. Reset the settings by opening the Configure Network screen for your network and selecting "Restore to Factory Defaults."
Configure Wireless
Your Xbox 360 console may not be properly detecting your wireless network if your modem is connected to a wireless router. Before connecting to Xbox Live the console must first be set to scan for the proper wireless connection. To access the wireless settings screen, open the "System Settings" tab in My Xbox and access the "Network Settings" option. Open the "Configure Network" option. Select the "Wireless Mode" feature and select "Scan for Networks" to locate your network and automatically configure the wireless settings.
MAC Filtering
Some routers use the Machine Access Control filtering feature as a security measure to protect your network from unknown devices. You won't be able to access Xbox Live if this feature is enabled. To find the console's MAC address, navigate through the Network Settings screen. Choose "Configure Network" and open "Advanced Settings." Bring up your router's settings menu by navigating your Web browser to the address "192.168.1.1." and add your console's MAC address to the MAC Filter list to resolve the issue.
Change MTU
Your network has a parameter called the maximum transmission unit which determines the largest single piece of data that will be sent or received at any given point in time. Your console won't connect to Xbox Live if the MTU is set too low. To change the MTU setting, access your router's configuration screen. Access the "Basic Setup" option and set the "MTU" to at least 1364.