DHCP failing accross the network

I am running a Win 2k server with 45 Win 98 SE workstations. I have DHCP installed and running now for about 8 months. Now the workstaions are failing to logon to the network using DHCP. If I use a static IP and subnet then the network sonnections are there. When the workstations are set to use DHCP they cannot even “ping” the server. I have tried to deactivate and reactivate the scope but this has not helped. I have no idea what could be causing this. There has been no changes to the server since the IP scope was first activated.