if your pc is still on the network, you can go to the router control page. I doubt they changed the default log in, because you would have already entered your password to be connected to the router so it should be
In there you can find the actual password to your network. Hope this helps
The default user name is admin and the default password is password. Obviously if the user name and password were changed, then these won't work. You'll have to either get them from Comcast or reset the device to factory defaults and re-start configuring from scratch.