Hi SarahS,
I have spoken with my colleague in IT, who has told me that he can see your email address as registered, so an attempt to register with the site has been successful and why you are now no longer able to register with your email address. It might be worth trying to login again with any details you originally used.
If you are unable to log-in successfully, I would recommend requesting a new password available. If you click on the Login link at the top of the page and then click on the "Forgot your password?" link.
Kindest regards,
Julia