stating username or password is wrong

when I try to retrieve my business email, or send emails an error message comes up stating "User name and/or password not valid".  i re-entered my password and username, which has   not changed, and the same message appeared.  i tried on my mobil phone as well as my ipad, and pc.  cache has been cleared, history, and all are up to date.  i still can not access my account.