Is my site hacked? please help

hi there i got a problem on my site, when i click on one of the category it shows “Catchable fatal error: Object of class WP_Error could not be converted to string in /home/content/90/11897690/html/wp-includes/kses.php on line 971” on that page nothing else, wheni scan iti got some bad results check here i don’t know what should i do…. any one can help me…the 971 lines code is here in kses.php file $string = preg_replace('/+/', '', $string);
please help me and tell me what should i do.
thanks