|I suspect you're asking about the key for the wireless network.|
If I'm correct, you'll want to connect to the router's management interface and go to the wireless configuration page. On there you should see a "passphrase" or "password" field. With luck, it show's the key in plain text. If it shows asteriks (Stars) then look for a checkbox to show it in plain text.
If you have stars and no way to show the passphrase in clear text, you'll have to enter a new passphrase and then change it on all your wireless clients.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.