How do I get my wirless router to work with my new isp modem?

I used to have internet with ATT but now have switched over to Comcast HSI. The wireless router I use for internet is Ativa brand wirless G router, and it worked fine. But now that I want to use it with the Comcast modem, it doesn't work. I guess there's something that I need to reconfigure in the router so that it will send out the signal, but I have no idea what to do. Does anyone have any advice, or do I have to contact the manufacturer. I can't find the box or the manuals but I do have an online account that has information like ISP, IP address, etc. When I check what's wrong with the internet, it tells me that a cable is not plugged in but I assure you everything is well plugged in. And other times, it tells me that there might be something wrong with my DNS code. HELP!!!How do I get my wirless router to work with my new isp modem?
I might need more information, but I'm going to go out on a limb quick because it's fun. Go to command prompt and run as administrator. Start %26gt; All Programs %26gt; Accessories. Make sure to run as admin. Type ipconfig/release and once that is done processing type ipconfig/renew. The simplest solution I can think of is that your router is still using the same ip address and dns server from your old modem, and doesn't recognized that it has been changed. If that doesn't work, feel free to shoot me an e-mail and I can help you out some more.