I agree with notfred and Justbrewit.
I would first swap the cable out. If you cannot then check to make sure it had not been chewed up or that it has something heavy on it like a table leg or that it has been rolled over by a chair with casters etc.
Next I would try a different port on the switch or a whole different switch all together.
Most switches these days are intelligent and they do not require all ports to run at the same speed. However most hubs are not buffered and do require everyone to talk at the same speed. If you are using a hub its time to let the poor beast go and get a switch.
If your main board has multiple Ethernet ports try a different one or use a separate nic altogether.
I would try disconnecting everyone else from the switch if possible and just have a connection to your broadband "modem".
Sometimes all you can do is the process or elimination till you find the culprit.
Oh yeah, don't forget to try rebooting your switch as well.
I wish to see things not as they are but as they should be.