If you have a laptop available, try this. Load inSSIDer onto the laptop. This is a wifi monitoring application, which will allow you to see how many other modems and routers are running nearby, and which ones, if any, are running on the same channel as your network, or on overlapping channels. Maybe your neighbor has brought home a new modem or router and set it up, which is now interfering with your network. This is the last freebie version of inSSIDer. It doesnt' show the 802.11ac networks that run in the 5 Ghz band, but there is a new pay version out now that does, and which runs on a normal 802.11 b/g/n laptop by reading network transmission headers in order to display all of the nearby networks, including the 802.11ac networks. I suspect that you're only concerned with the 2.4 Ghz band at this time, so this version will do the job. If you determine that network interference from nearby modems or routers is an issue, maybe you can also determine if there are any channels available which don't have as much interference.
Okay. You'll need to provide more information then that.
Is it connected directly to your modem? Do you have it connected by Wi-Fi?
Can you see the network on your Xbox and attempt to connect to it? If so what is the error message?
Can you not even see the network on your XBox?
Details. We need details if you want help.