Seems there are a lot of reasons you run into this distressing problem with Magento. So no one thing is certain to solve everyone’s problem. So here is a triag/list of things to do until the problem is solved.
REMEMBER TO RESTART APACHE AFTER EACH CHANGE
sudo service apache2 restart
Uncomment to enable Error Reporting in Magento’s index.php and at the same time increase the memory limit for PHP, (since this is the most common problem you can quickly do it here)
error_reporting(E_ALL);
ini_set(‘display_errors’, 1);
ini_set(‘memory_limit’, ‘256M’);
Disable Compiler via SSH
Check status
php -f shell/compiler.php — state
Disable Compiler
php -f shell/compiler.php — disable
Clear all files
php -f shell/compiler.php — clear
You will probably see a message displayed something like this:
PHP Fatal error: blah-blah-important-message-here ///lib/Varien/Http/// on line xx
Check PHP Memory Limit
sudo vi /etc/php5/apache2/php.ini
128mb is normal
(if a new server) Install PHP-CURL
sudo apt-get install php5-curl