There are several possible reasons why your Internet connection might suddenly stop working. If you were on dialup, the most likely scenario would be noise on the phone line, but since you have a high-speed cable connection, we can rule that out.
In a recent Ask Bob Rankin article Do Computers Get Tired? I addressed the subject of electronic devices that fail at random times, and gave some scientific basis for occasionally turning the device off and then back on. I have a cable modem as well, and have found that sometimes a slow or dropped Internet connection is restored simply by unplugging the cable modem, waiting 30 seconds and powering it back on. Not pretty, but it works. If the problem is happening every day, though, it might be better to replace the modem.
There could also be a software-related issue which is causing your Internet connection to fail. If you have other computers on a home network, and they have no trouble getting online, then I'd cast a wary glance in the direction of your firewall. Firewalls are designed to block certain Internet connections, so it's entirely possible that a bug in the firewall software is erroneously shutting down ALL network connections. You may even have told the firewall to do this without meaning to.
Open your firewall's configuration screen and check to see what programs are being blocked from connecting to the Internet. If nothing obvious appears to be erroneously blocked, try shutting down or uninstalling the firewall software, then reboot and see if the problem persists. If that fixes the problem, consider ditching the software-based firewall, especially if you have a router between your computer and the cable modem. Routers have hardware-based firewalls built in, which makes firewall software superfluous for most users.
To uninstall the firewall software, click on the Start button, open Control Panel, then Add/Remove Programs. Find the firewall in the list and click the Remove button. Note that your firewall may be bundled with an anti-virus of internet security package. If that's the case, click on that package (ex: eTrust EZ-Armor or Norton Internet Security), and make sure you select ONLY the firewall for removal, leaving the anti-virus protection in place.
If none of those things helps, report the problem to your Internet provider. If the problem has something to do with your modem or the cabling in your neighboorhood, it might be affecting your neighbors as well.