boot -s
this should get you to a root prompt without loading the network drivers. Exactly which part of the network boot is failing, eg you could be hanging on nfs and ip would still be up. Can you ping the sparc, if so try telneting in. If you post where it hangs it would be helpful. I have not seen a non-bootable cd on a sparc, which make and model is it and how did you upgrade?
thanx,
bill