Cannot see machines in Network Places

Here’s the situation… Windows NT 4.0 running on PDC. All workstations are running Windows 2000 Pro. For the past 3 years never had any problems going into My Network Places and viewing all 200+ machines and servers on the network. Today all of a sudden all machines can only see the same 3 computers on the network, but can ping ones that they don’t see without a problem. Any ideas on what could have caused this and how to fix it?