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Default Network Key/Wi-Fi password. Can be seen on the ticket sometimes located behind or at the bottom of the modem/router. It is usually labeled.
'WPA Pre-shared key' or 'Wi-Fi password'
If default password won't work you can read this thread to see basic troubleshooting steps for Wi-fi network.
Let us know how it will go
You will find your network key on the bottom of the modem. Look for a string of 8 numbers - sometimes referred to as a password. You can also get the network key by logging into your modem putting 192.168.1.1 into your browser and logging in using user name "admin" and password "admin" then click on advanced then go to the wirless section where you will find it listed.
Here's a helpful guide: Forgotten Your Wi-Fi Password?