Assuming that the systems were indeed up and running, do these diagnostic
steps:
1) Reboot the system.
2) Check that cabling is correct.
3) Try another system to see if other systems can be seen.
4) Check that terminating resistors are in place.
5) Try reseating the Ethernet board. If that doesn't work, try a new board.
6) Check your IP address to see if it's valid.
7) Also, check your domain name.