Network Cable Unplugged...?

My computer frequently says "a network cable is unplugged". This disconnects me from the internet and whenever I use my Xbox 360 for Xbox Live, I get signed off because of this. Does anyone know what the solution to this problem could be? All cables are plugged in and I've made sure that i heard the 'click' for when they're in all the way. I use a modem connected to a router, and the pc and xbox are connected to the router.