wow u really have to give more info??
are all on the same network with the same ip range ?
are you sure it is network cables and not power cables?
what is the results for the "ipconfig /all" command if it is windows machines?
and and and...
check the default gateway, it should be the same on all the PC's if they're in the same Network.
I have encountered one unique problem with a few networked computers in my lab. whereas most computers can access the internet, some do not- though the cables are fitted!!
The thing that should be checked is the ethnet (NIC) adoptor and you should make sure that all the cables connected to those computers are in a good condiotion, because some times when the cable have been used for a long time they tend to loose strenth and course signal fail from the main server.