Network Computer Not Shown in Workgroup

Hi, my whole company is running a networked workgroup. We have XP Home, XP Pro, Vista, Mac, and printer as well. For the past 2 years all this PC is shown in the "My Network Places -> View Workgroup Computers". But Suddenly everything is disappear in the workgroup computers. The only thing shown is the mini - server. All other computer is not shown here. However, all can be ping. Please help. I don't know what happen suddenly.