One PC suddenly not connecting?

Hi all,

Got a slight problem with my network and not sure what it is, was hoping someone on here may be able to help. Basically, I have a couple of computers connected up to my home router (Sagem router provided by sky broadband) using a wired connection, and also connect laptops, iphones etc. up wirelessly. Everything else apart from of one of the wired computers is working fine, but the computer in question just won't connect to the network, says a network cable is unplugged? I have tried changing the cables at the either end of the wiring setup and they seem fine, can't test the main cable though as it's too long and attached to the outside of the house. I have also looked for an update for the ethernet part of the motherboard, found it, but can't download it from anyway and the product page for the ABIT AB9 Pro motherboard is useless, can't download anything from it? Outside of these two options I'm not too sure what it could be other than the main cable unless I am missing something really obvious? Have tried both network connections on computer as well so not that. Unless I can solve this quickly I am going to have to purchase a wireless network card to put in, need it connected back up asap.

Any ideas are welcome.

Thanks,
Tom