DHCP problem: clients given NACK for dhc

Clients are losing their connections: seems that they are not finding dhcp server.
WINIPCFG on client reveals dhcp server as 255.255.255.255

This happened straight after a server reboot. The wierd bit is that the clients appear to be requesting the server’s ip address.
DHCP log after reboot shows:
11,12/14/00,18:16:53,Renew,x.x.x.54,drawing,005004361976
15,12/14/00,18:16:54,NAck,x.x.x.8,drawing,005004361976
i.e. client called drawing ip address .54 is renewed and 1 second later receivesa NACK but with its address shown as x.x.x.8 which is the address of the dhcp server.

Any suggestions/fixes appreciated.