Cannot logon to incoming mail server?

I am unable to log on to the incoming mail server . My ymail account has functioned without problem for years. Now suddenly It won't log on. I am using MS Office Outlook 2003 SP3. I ran the "Test Account Settings" in Outlook. It returned this message "Outlook could not logon to the incoming mail server (POP3) Please verify settings for User Name, Password and Email fields" Please tell me what they should be.