Blog header with error call_user_func_array()

My blog currently lists the following error message across the top:

“Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘check_wordpress’ not found or invalid function name in /data/23/1/6/54/1984217/user/2168224/htdocs/blog1/wp-includes/plugin.php on line 405”

Network solutions says that this is a problem with the open source software I’m using (?) Any help is appreciated. This blog has been down most of the week due to problems on the back end.