We're sorry to hear this!
As a first step in trying to fix this issue, we suggest double (and triple!) checking your details. The password is the one you set up when opening your account.
If you're still having difficulty, we suggest clearing your cookies or using another browser.
Resetting your password may also help you sign in - when prompted to put in your details, click the 'Forgot Password?' link and enter your email address. You'll be sent an email with instructions on how to reset your password. Keep an eye on your spam and junk folders as the email may end up there.