If you've changed the router then you've changed the 'PC' that the cable modem thinks it's talking to.
If you have a stand alone cable modem (SACM) then all you need to do is power it off and then on again. But, if you access the internet through a set top box (STB), then you'll have to go through the process of re-registering the new device.
More details here:
http://homepage.ntlworld.com/robin.d...tips/swap.html