If you are using the same cable to connect with the external network card, then it points to the internal lan port being faulty. This is poss due to inserting/removing the cable, no matter how careful you are, this can cause damage over time.
If the LT in under warrenty, then you can try to get them to sort it, this would poss require a replacement mobo or maybe just re-attaching the port to the board. If you do decide to send it back for repair, then make sure you have a working backup of all your important files.
If this is not covered by warrenty, then it maybe better to stick to using the external network card.
I would suggest that you check that all the other ports are woking, but I think that you will be lucky, and it's just the lan port that is broken and the rest of the LT is fine.
Hope this helps.