Instructions
Reset your router. On the back of a wireless or wired computer router will be a small button labeled "Reset." This button will likely be too small to press with your finger, so use the tip of a pen or pencil to press and hold it in until your router resets itself. If your router was not broadcasting an IP address properly, this will fix the issue and allow your home network to get back onto the Internet, which will also pave the way for your Xbox 360 to go online.
Unplug your modem. After a minute has gone by, plug it back in. If the Internet connection has gone down throughout your home, not just with your Xbox 360, the first thing you should try is to always unplug and plug the modem back in. Modems are designed to run for 24 hours a day, seven days a week. Every so often it will need to be rebooted to overcome errors like a failure to validate an IP address.
Change which USB port your Xbox 360's wireless adapter is plugged into. The back of the Xbox 360 has multiple USB ports, similar to the layout of the back of a computer. Try unplugging your wireless adapter and plugging it into another USB port. Then, try reconnecting to your home's wireless Internet connection. This won't fix the problem with your original USB port, but it will allow your Xbox 360 to get onto the Internet so that you can enjoy Xbox Live and all its various services and features.