Something on your network has changed. The two most likely candidates are:
1) Network configuration. New switch, router, or policy that is now blocking ports.
2) Someone has (possibly unknowingly) enabled a DHCP server on the network and it's capturing your NebBoot packets.
It turns out that the central university IT department (in their INFINITE wisdom, ha) had decided that enabling NetBoot and PXE on their network is BAD. They hadn't bothered to tell anyone about it, either. Oh, what a fight it was to get a port on their switch marked as "trusted" just so I can have NetBoot working.
Sorry I can't be of any more help to you.
I posted this before and is well worth looking at especially if you have made custom ethernet settings or done any trunking on your netboot server. Must be the MTU frame rate that dictates the functioning of the Netbbot service.
This might work for some:
I had the same problem of the netboot image not mounting/opening, spinning globe then booting back to the Mac HD on the client and it was only when I remembered that we had manually experimented with jumbo frame rates and the MTU settings for our 32TB RAID in the Hardware settings for Ethernet ports on our servers. I turned the settings back to configure "Automatically" and 1000baseT on, then the whole NetInstall and Neboot was working perfectly again.