If you are unable to find the wireless network, you can try a few different things. If your router is far away from the device you're trying to connect to the internet with, you may need to move the router. It should be away from walls or metal surfaces as well as devices that may cause interference such as microwaves or wireless phones. If this option doesn't work, you might need to install more routers or signal extenders to ensure that the Wi-Fi signal reaches across your house.
The problem could also be an internal hardware failure in the router. You may need to replace the antenna or the Wi-Fi card. There is additional information listed in the [[Apple Airport Extreme Model A1408 Troubleshooting|troubleshooting page]].