Can anyone help please?

Went to check my emails a couple of days ago (Outlook Express) and for some reason OE is asking me for my username and password (which are already filled in on the popup box. I press OK and it refuses me.

There was a problem logging onto your mail server. Your Password was rejected. Account: 'pop.1and1.co.uk', Server: 'pop.1and1.co.uk', Protocol: POP3, Server Response: '-ERR authentication failed', Port: 110, Secure(SSL): No, Server Error: 0x800CCC90, Error Number: 0x800CCC92

The above is the message I get.
Why is it doing this to me after 3 years of no problems?

This is Al Clegg - News at 10.00 a.m. - Dumaguete - rather p****d off - handing back to the studio.

Al.