Probably the drivers. Right-click the start menu button, and click on Device Manager. Look under Network adapters. Is network adapters there? (If you also have WiFi, it should be listed). Is the Ethernet driver listed?
If it is: Double click on it, and go to the drivers section. Click on 'update driver' if it is avaliable.
If it isn't: Click on action at the top, then try 'Scan for hardware changes.' If that doesn't work, try the 'Add legacy hardware' wizard instead. Or, try and find the drivers online. If you have to, do this from another computer and copy it over with a usb drive.