This could be due to a number of reasons. To troubleshoot, try connecting another device in your home to your wireless network to make sure the network is working. If not, you may need to restart your router/modem. Make sure your printer is within the range of your wireless router, and that it is not placed near a microwave, cordless phone or large metal objects.
You may need to check your router's MAC settings - these could restrict new devices from being able to connect to your printer. If settings are in place, check the MAC address of your printer and register it with your network (check guide linked below on how to find the address). Additionally, you may have to input the name of your network (SSID) manually if it doesn't appear, or check to make sure the WEP/WPA keys are entered correctly.
For more troubleshooting info for network connectivity, visit the printer's [http://files.support.epson.com/pdf/nx420_/nx420_qr.pdf|user manual.]