First of all, if you have another network cable you should replace the cable you are currently using with the spare one. If the problem doesn't go away, then there could be a modem/router/switch malfunction. Try plugging the cable into different slots (if you have more than one lan slot on the device) or reset the network device or replace it. If the problem is still present, check the network card driver. Try to disable and enable the network card from Control Panel -> Device Manager (for Windows OS). Uninstall the driver and reinstall it or update the driver if necessary. If this doesn't solve your problem try connecting another computer to the network. If other computers work, it means that your network card is defective so you must replace it.
If you don't have a spare network cable, check if the cable is properly connected to the modem/router/switch and to your network card. If it is, unplug it and check the cable for faults. It could be interrupted or the wires could be cut near the connectors. If this is the case, replace the cable.
This happened to me the other day and I discovered that the network cable had the wires cut at one end, near the connector.
