can't access domain or dashboard

Hi,
I was building my website just fine and then tried to insert a text widget from mailchimp which worked at first but now I can’t access my domain by going to the url or the dashboard by going to http://www.domain/wp-admin
I’ve cleared all my cache and cookies. I got to the login page by putting in domain/wp-login.php and it gave me a whole set of warnings about headers not being changeable on different lines?!?

can anyone help please?

Thank you