Do you have an internet connection? Xbox live gold?
Try rebooting the whole console as well or power cycle
press and hold the Xbox button until the light behind it turns off (about 5 seconds), which means the console is off.
After the console has shut down, unplug the power cord and wait 30 seconds.
Plug the console back in and turn it on using the Xbox button. Power cycling will reset the network card and clear the cache.
