You don't mention which operating system it has. Anyway, open control panel and network connection s ( XP ) or network and sharing ( Vista ). See if the LAN icon is there, if it is, right click it and enable.
If it is not there, then right clcik my computer, properties, device manager. See if the lan is listed under network connections and if it has any yellow warning symbols. If it has, right click and update the drivers.
If it is not listed in device manager, it will need to be turned on in the bios. Do you know the make and model of the motherboard or is it a branded machine.