Internet Problem

I’ve got three base units which are connected on the same network.
All three computers have lost there internet connection and all three said that a network cable had been unplugged, which it wasn’t. I then tested the cat-5 internet cables with a cable tester and on different computers and they worked fine.
On the back of the base unit only the green light is showing, and it flashes every now and then, but no orange light.
When I go into Network Places it shows absolutely nothing it doesn’t even see the network card it shows nothing.
Has the network card bust? Or could it be a virus spreading the network as this has happened to three computers already.