again, this is pretty easy to debug. example:
login to your server and:
find Apache error log location:
first get the configuration file:
~/#: httpd -S
*:80 is a NameVirtualHost
port 80 namevhost example.com (/etc/httpd/conf/httpd.conf)
look for ErrorLog path:
~/#: grep ErrorLog /etc/httpd/conf/httpd.conf
use tail to read the latest log entries:
~/#: tail -f /etc/httpd/logs/error_log
~/#: cd /var/log/nginx
~/#: tail -f error.log
Magento internal logs:
if you are not sure where is the shop installed:
~/#: locate app/etc/local.xml
if you are the owner or you know where is the shop:
~/#: cd /var/www/html/shop.com/var/log
~/#: exception.log system.log
~/#: tail -f system.log
open your browser and go to your admin page (or any page that fails or blank, etc), click it few times, press F5, etc.
and see your error log fills-in with error messages.
but obviously you have to flush the cache storage and clear opcode cache as well,
dont forget about memcached and redis :)
p.s: This example uses the default options.
Your server may use different configurations and paths.