Could not see Appletalk Zone

On any network Macs at my site(Ethernet) I used to see 2 zones: FUSD (remote) and FOHI (Local). Now, I can see only “FOHI”, the Mac displayed message “There is problem with network router”.
How can I correct the problem?

Site info: a Cisco 2600 router config with Appletalk enable, it had been working and nobody changed anything (It’s used for Internet access).
All function are working fine.

Macs are connected to the Ethernet backbone by NIC 10/100BT and able to see
2 zones before.

Thanks for all your help.