If you're connecting devices directly to your modem, you should plug the device in, turn the modem off for a couple of seconds and turn it back on. Wait a minute or so and you should be golden.
If that doesn't work, then your problem might actually be the result of your cable company. The very first thing some cable providers do is register the MAC address of the first network device you connect with (a somewhat unique number assigned to your network device). This could be a computer, router, Playstation etc... These items all have MAC addresses.
Then, they block any devices that do NOT have the same MAC address as the original network device you connected with. You have to call the cable company and register the new MAC address with them.
