Has the problem just started on an old account (where you could log in with the same credentials previously) or is the account newly created? Is this with webmail or an email client? If email client can you log in with same credentials via webmail?
If you are 100% sure the username & password are correct then check that caps lock isn't enabled, if possible try entering the details via an onscreen keyboard.
Virginia/Doctor9fan. If I have helped please give me Kudos.