Check the cable modem itself. Unplug power from the modem and let it sit for a few minutes. Plug it back in and make sure all the activity lights come back on. If the lights don't come back on the modem, make sure it is not set to 'standby' then call the ISP to do a line test and possibly even replace the modem. If the lights look good on the modem, then reboot the computer and see if it is able to obtain a valid IP address. If you still can't get an IP address, verify the connection between the modem and the PC is good (unplug and reconnect it and use a different cable if possible), then reinstall the drivers for the NIC. If all else fails, you may need to replace the NIC in the computer.
edit: Also, when you try to renew the address, does it actually say that it was unable to complete the operation or does it complete but give an invalid (all zeroes) or Automatic Private (169.254.x.x) address?