If the Apple TV cannot find a WiFi network when there should be one in range, this may be due to an automatic connection failure or network information mismatch.
Automatic Connection Failure
If the network you are trying to use isn't listed, you can scroll to the bottom of the list and select the 'Other...' option to enter the name of the network manually.
Network Information Mismatch
If you are still unable to connect after correctly entering your network's name, please restart your wireless router before attempting to connect again.
If unsuccessful, you can go into the network settings by typing 192.168.1.1 into the browser of a computer with access to the network and log in.
Make a note of the SSID in the router's settings so you can enter it into the Apple TV's manual connect option.