The reasons for this may be:
1. The network information may have been configured incorrectly.
2. The server has been configured properly for a network, but is currently not on the network.
3. You may have booted with the "config=Default" option which effectively takes you off the network because the default configuration does not include a network driver.
Solution:
Your server will eventually boot after a very long series of timeouts, but you can overcome this by resetting its network configuration information to a default state.
The best way to do this is to boot in single user mode and replace the file.