No login prompt

I have a machine running Win 98 and it runs smoothly. But twice it happened. The enter network password screen does not show, instead the windows logon screen comes up. We are having a NT network with 1 PDC. the first time after 2 days it came back automatically but this time i am in trouble as after 2 days it hasnt come back. I tried registry settings HKey/localmachine/network/logon to default.

Any help please..