Release and renew the IP. If this doesn't work, you should reboot, or power cycle, the modem. Usually this means unplugging the AC power either from the wall or the back of the modem, waiting 30-45 seconds, and then plugging it back in. Some modems have a reset switch which can also be used. Power cycling the modem should be one of the first steps when troubleshooting a "dead" connection.