Via Admin Panel
Navigate to System > Tools > Compilation page and click on Disable button
Navigate to System > Cache Management screen and use Flush Cache button.
Via SFTP
To disable compilation in Magento, edit includes/config.php. At around line 28, uncomment the first line and comment out the second:
#define('COMPILER_INCLUDE_PATH', dirname(__FILE__).DIRECTORY_SEPARATOR.'src');
define('COMPILER_COLLECT_PATH', dirname(__FILE__).DIRECTORY_SEPARATOR.'stat');
Via SSH
php -f shell/compiler.php -- disable
php -f shell/compiler.php -- clear
Check the status:
php -f shell/compiler.php -- state
Compiler Status: Disabled
Compilation State: Not Compiled
Collected Files Count: 0
Compiled Scopes Count: 0
Clear the cache:
rm -rf var/cache/* var/session/*