Domain Navishes from log on screen

I am a Lan Adminsitrator for a Small firm.I am facing a problem in Windows NT Server 4.In one or two weeks the Domain option from the Log on screen vanishes.This is a member server with network printers and files for the users.There is no log on Event viewer or anything.It not even allows to log on locally using admin account.Can anyone tell me why this problem comes and what is the solution for it.