Unplug the cable modem and shutdown the pc.
Wait a full 30 seconds. Then turn the modem back on and wait for the "sync" and "rdy" lights to go solid. When they do power the pc back on.
This should then let you connect. If you are using win98 or ME, you may also need to go to start-> run -> winipcfg
then press "release all" then "renew all"
hth
MB