Lan Cable unplugged

[Andrew]

I was working good with the boardband connecting(DSL
Modem) with my PC directly.
But after I change the router to join another pc and
mobile. the original pc shows the Lan connection with red
X said ' Lan cable unplugged" no matter how I fix it, I
cannot make the connection probably.
But I have try the connection with the mobile pc, It
works good.

Please help to slove the problem.

THanks