Site Down: Cannot modify header information – headers already sent by

Dear Sir

My site was running ok till last saturday. I was seting up the site using your theme following the readme. But now from this morning the wp-admin page is not opening and not the site.

the error log showed something like this. Can you please help me ?

[28-Dec-2015 12:11:18 UTC] PHP Parse error: syntax error, unexpected end of file in /home/lighthou/public_html/wp-includes/class-wp-theme.php on line 506
[28-Dec-2015 12:11:41 UTC] PHP Fatal error: Class ‘Walker’ not found in /home/lighthou/public_html/wp-includes/class-walker-page.php on line 17
[28-Dec-2015 12:11:46 UTC] PHP Fatal error: Class ‘WP_Widget’ not found in /home/lighthou/public_html/wp-content/plugins/revslider/includes/widget.class.php on line 10
[28-Dec-2015 12:49:17 UTC] PHP Warning: Cannot modify header information – headers already sent by (output started at /home/lighthou/public_html/wp-content/plugins/contact-form-7/includes/functions.php:1) in /home/lighthou/public_html/wp-login.php on line 387