Awful question, far too vague. If you want a real answer, provide details.
Did you move the site? Does the site URL stored within WP differ from the domain you're using to access it? Did you edit core (in which I say: WHY!)? OR wp-config.php and forget a closing bracket or a semi-colon? There's way too many reasons your site could be down...first and foremost, turn your errors on (via functions.php) and go from there
ini_set('display_errors', 1); error_reporting(-1);
& also go to wp-config and add: define( 'WP_DEBUG', true );
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…