If the Ethernet cable does not seem to be the reason why you cannot connect to the network, check the laptop's network settings. In Windows XP, open the Control Panel and select Network Connections. Open Local Area Connection and check the status of the connection.
In Windows Vista, go to the Network and Sharing Center and select Diagnose and Repair from the left pane and wait for the results.

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If the Local Area Connection is listed as Disabled, right-click it and choose Enable.
If it says that the network adapter is unplugged and you have ruled out a problem with the Ethernet cable and port, the laptop's network adapter may have failed. Try using the same Ethernet cable on another computer. If you are able to connect to the network on another computer using the same Ethernet cord, contact the laptop's manufacturer, a computer repair technician or replace the network adapter yourself.
If it says Limited or No Connectivity, there may be a problem with the laptop's modem or router. It might also mean there is a problem with the laptop's TCP/IP settings. For detailed steps on troubleshooting your laptop's TCP/IP address, check out the Bright Hub tutorial Troubleshooting TCP/IP & IP Addresses on Networks.