We have another computer connected to this one in a local area network. The other computer has suddenly stopped being able to connect to this one (to access the Internet for example), and always says "network cable unplugged", when in fact it isn't. I have checked up in the roof to make sure the connection up there (we needed an extention cable to reach the other comp), but it all looks fine. Both ends of the cable are firmly inseted into the back of each computer. Why might it be saying this?