Make sure the Ethernet cable is connected to the network. Check that this system has an entry in the NIS ethers(4) map or locally on the boot server. Then check the IP address of the server and the client to make sure they are on the same subnet. Local /etc/hosts files must agree with one another and with the NIS hosts(4) map.
If those conditions are not causing the problem, go to the system's PROM monitor ok prompt and run test net to test the network connection. (On older PROM monitors, use test-net instead.) If the network test fails, check the Ethernet port, card, fuse, and cable, replacing them if necessary. Also check the twisted pair port to make sure it is patched to the correct subnet.