Network Cable Unplugged usually indicates:
a) One or other end of the cable isn't plugged in properly to the RJ45 port.
b) The wrong type of cable has been used - "crossover" instead of "straight-wired" or vice versa.
c) The ethernet cable is damaged.
d) One or other of the RJ45 ports is damaged.
If you are experiencing the problem with *ALL* computers connecting by ethernet cable to a router, I would suspect that the router's built-in network switch has failed.