Why is my pword suddenly being rejected by the server?

I have Outlook Express for POP3 account. I changed no settings but for no reason, when I want to connect to Outlook Express to open up my e-mails, a window pops up asking me to verify my username and pword, so I click OK and then it just keeps asking me over and over again. I can't get authorization from my server and I don't know why, because I changed no settings.