Nginx gives blank pages - 500 error

After resizing my Server's RAM and rebooting, every thing was normal, but the website on nginx gives blank white page, i didn't detect any "Error logs"

server {
    listen 80;
    server_name mysite.com;
    access_log /var/www/mysite/application/app/logs/access_log;
    error_log /var/www/mysite/application/app/logs/error_log;
    root /var/www/mysite_ch/application/web;

    location / {
        rewrite ^/version-(\d+)/(.*)$ /$2 break;
        try_files $uri /app.php?$args;
    }
    location ~ ^/(app|app_dev|config|apc-.*)\.php(/|$) {
        fastcgi_pass   unix:/var/run/php5-fpm.sock;
        fastcgi_split_path_info ^(.+\.php)(/.*)$;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param HTTPS off;
    }
}

for the php file:

-rw-r--r-- 1 www-data www-data 750 May 24 16:20 /var/www/mysite/application/web/app.php

edit: I used Chrome console to check the returned code, which is "500".