Magento has a ‘compile’ feature which is supposed to get your site to run much faster, what it will most likely do instead is completely break your site, so that you can’t even get in it turn the ‘feature’ off!
Don’t panic a quick edit of this file, will get your site up and running again:
Edit the file via FTP or similar and make sure the the following lines are commented out like this:
#define('COMPILER_INCLUDE_PATH', dirname(__FILE__).DIRECTORY_SEPARATOR.'src'); #define('COMPILER_COLLECT_PATH', dirname(__FILE__).DIRECTORY_SEPARATOR.'stat');
Once you have completed the edit your site should work again!