Problem accessing one account on webmail

Hi

I can't get into one of my email addresses on webmail. On the account details it has the following message:

"The entered credential or authentication information does not work or are no longer accepted by provider. Please change them."

I've had a look at the settings, but can't see anything wrong. I can't remember the last time it worked because I only use webmail occasionally to check older emails, but that address is working fine on my email client on the computer. Could I get some help please?

Thanks

John