If this is your first time logging in, you may have made a typing error when registering. You have not received an activation email for your account.
If this is not your first connection, it is possible that you entered an address which is not that of your account or that you made a typing error. Don't hesitate to use the "Forgot your password?" rather than multiplying connection attempts because after 3 failures, your account will be temporarily blocked.
If your account is blocked, contact support via the contact form provided for this purpose.
Your connection may also fail if your browser has been configured to refuse "cookies". These are essential to open your session and give you access to the courses in which you are registered.