Was it ever changed from its default value ?
Is this a Comcast supplied gateway device ?
If so, see this;
You can also view and reset your Wifi password via your online account.
View and Change Your WiFi Information Online with My Account
1. Visit www.xfinity.com/myaccount and sign in with your XFINITY username and password.
2. Select Internet.
3. Select Edit next to WiFi credentials. Here, you will see WiFi network name and password displayed.
4. You can change your WiFi name and/or password by selecting Edit, entering something new and selecting Save.
Note: This online tool supports all current XFINITY Wireless Gateway models.