Computer won't connect to internet. Network cable unplugged?

At my house, we have a desktop and three laptops. The internet cable is plugged from the wall into the modem, the modem is plugged into the router, and the router is plugged into the desktop computer. The wireless internet connection on all of the laptops is perfect, but the desktop is suddenly not receiving any internet signal. A note comes up that says a network cable is unplugged. I don't know what happened. Any suggestions?