A sequential power cycle is the first step in clearing connection problems, and the order and timing of the steps is important to success:
1) turn off the Xbox then
2) unplug power from the router (Some folks have a combination modem-router AKA 'gateway' or only use a modem with no router. If either, skip steps 3 & 6).
3) unplug power from the modem.
4) Wait at least 5 minutes then replug the modem/gateway power.
5) Wait 'til the light on the front of the modem/gateway which says BROADBAND, ONLINE, READY, CABLE or DSL is on solid (not flashing), then plug in the router. If you have a gateway instead of a modem and router or a modem with no router, then when that light says it's ready, skip to step 7.
6) Wait until the WiFi light on the router is on; you may not use WiFi, but it's still a good indicator your router is finished booting up.
7) Turn on the Xbox. For more background on Ports, see whatismyipaddress.com/port
If the problem persists, please send us the info we need from http://forums.xbox.com/xbox_forums/xbox_support/f/9/t/155724.aspx so we may work on this further.