Which distro are you using? Have you tried configuring your ethernet card? In Gnome, you could do this with via System > Administration > Network - make sure that the card is enabled, and set up to configure itself automatically using DHCP (assuming your router does this, which it probably does). KDE and other desktops will have similar configuration tools. Then, as Tredegar says, you could try restarting the laptop with the cable plugged in.
If this doesn't work, please post the output of:
- The section of lspci -vv that shows your ethernet card