Instructions
Turn off your modem and router and then restart them in sequence. In many cases this will fix a connection problem, but if it doesn't work, proceed to step 2.
Use your Xbox controller to select the "My Xbox" option from the main menu screen. Then select "System Settings" from the list of options.
Select the "Basic Settings" tab and then select your particular wireless network from the list of options. If you can't find your network in the list of options, select "Specify Unlisted Network" from the list and manually enter your network in the text box on screen.
Enter your network security password (if you have one) and then select the connect option to try and connect to the network. Your connection problems should now be fixed.