There are two common issues.
Firstly make sure you are logging in through the log in link and not through the sign up form.
Secondly please make sure that arrive on this page and enter your email address and password.
Finally If after entering your password you get the following fault
It means you have used an illegal or incorrect symbol or character in your email. For example you have used ^ or a % or something which is not part of an email. Check that you are using @ rather than any other symbol in the middle of your email.