|I'm assuming you're NOT using a wireless network connection between the computer and a wireless router that the cable modem is connected to. |
If you're connected to a router between the computer's network adapter and the cable modem, the network cable from the computer must be plugged into a network port other than the one labelled WAN on the router, and the cable modem must be connected to the WAN port.
When a poor connection or no connection to the internet happens, high speed modems usually DO NOT reset themselves to a better internet connection after the reason for the poor or no connection has gone away.
Try re-starting the cable modem.
If it has a switch, switch it off , switch it on.
If it doesn't have a switch, unplug the DC power connection at the modem by pulling out it's plug, not by pulling on the cord, plug it back in
Wait a short time until the leds on the front of the modem indicate you have an internet connection.
You may also need to Restart Windows. .