Outlook cannot connect (UserID/Password) not excepting

I have a exchange server I had configure it 4 months back since that time it was working fine but from yesterday I am facing one problem, All of the services of exchange are online even the mail boxes store is mounted and there is no error in the event log, but when user is trying to connect to exchange server by outlook 2003 it ask for User id and password that time it keep on prompting for user ID and password, Even though it recognizes that e.g. abc account mail box is on xyz server but at the time of checking the UserID of that user it keeps on asking user ID and password but not excepting it.