Well I have solved it.
I went back through the event log on the server to see what had happened between when my POP3 access to the email server worked and when it didn't. And I found that my server had a hard disk error apparently on the Mailbox!!
I ran CHKDSK /R and rebooted the server.
This seems to have fixed everything. No sign of any ill effects on the mailbox or loggin in to the POP3 server at all.
Peter