If your ethernet network card has apparently stopped working, or you are having other network-related problems, there are a number of diagnostic tests you can perform to find the problem.
Step 1 - Check Your Connections
It may be that the network card is fine and the ethernet cable has become loose. Make sure that the cable is securely clipped in at both ends. If you have a spare ethernet cable, you could try swapping the cable for that one, in case the old cable was damaged somehow.
Step 2 - Check Your Network Adapters
Check your network connections. Go to Start Menu > Control Panel > Network and Sharing Center > Manage Network Connections. From there you will be able to see all the network adapters that are currently on your computer and the status of each one. The status column will show the name of a network that the adapter is connected to, or it will show "unplugged," "not connected" or "disabled." If your network adapter is "disabled," right-click on it and enable it via the context menu.
Step 3 - Check your Network Drivers
Go to Control Panel > System and Maintenance > System > Device Manager > Network Adapters. Here you will see the drivers for your network adapters, right-click to view their properties. Make sure that there are no conflicts and that the correct drivers are enabled. Uninstall and reinstall the drivers if necessary.
Step 4 - Replace the Network Card
If it is a hardware problem, then the only practical way to know for sure is to replace the network card and try connecting to your network with another network adapter. The most hassle-free way to do this would be to use a USB ethernet adapter and then just disable the drivers for your old network cards via the Device Manager.
Step 5 - Ping Your Network
You can try pinging other machines on the network to test whether or not you have a solid connection. Pinging is simply sending a packet of information to another machine and waiting for a response which shows that the connection is sound. To do this, go to Start Menu > Run, in the run dialogue box type "cmd" without the quotes. Then in the black DOS box that appears type "ping <address>", without quotes and replace <address> with an IP address or the name of a computer.
If you have a router through which you connect to the internet, it's IP address is usually 192.168.1.1. If you can't ping this, then you won't be able to ping anything else that connects to you through that machine. If you have other computers on your network which can connect to the hub router, try using them to ping the computer that is having connection problems. Use the computer's network name if you don't know it's IP. Its name can be found in the Network and Sharing Center. If it says "request timed out" then the connection is down. If you are successfully pinging other machines on your network but you can't share files or resources, then it may not be a hardware problem.