|That almost certainly means that|
1) You don't have a network card installed on your pc
2) You have a network card but haven't installed the driver
3) Your network card has died
4) Your network cable is unplugged
Go to device manager, right click mycomputer -> hardware -> device manager and expand the network adapters node.
If there is no network adapters node then it is most likely 1) or 3) above.
If there is network adapter listed and it has a yellow exclamation mark on it then it is most likely 3) above i.e no driver.