How is the connection made? Is it by an ethernet cable or via WiFi? If it's WiFi, is the router's SSID shown among the available connections (and are the SSIDs of your neighbour's routers showing up?)
Try a different ethernet cable; try a different port on the router; if the ethernet connection to the PC is via a pci card, check that it has not become lose from the motherboard. Normally at the ethernet connection to the PC there will be two LEDs (with the cable connected, one should be permanently on, the other flashing when data is transmitted).