Invalid user name or password

I am unable to download new emails as I keep getting the message “Invalid user name or password”. I have not changed my user name or password so why has this happened please? It also tells me to check my settings but as I don’t know what the settings should be what am I supposed to check? Many thanks.

Which extension are you referring to here?
Is it gmail, live, yahoo, hotmail or private like your company domain name?

Thank you for responding.