Magento offers users the ability to compile their site code to improve performance, but if you have any bugs or issues with your site it can cause problems. Big problems; like taking down your site and locking you out of the Magento Admin Panel kind of problems. After enabling compilation you may find you've been kicked out of your site and you see error messages like this...
Warning: include_once(Mage_Core_functions.php) [function.include-once]: failed to open stream: No such file or directory in /home/mysite/public_html/app/Mage.php on line 36
It may seem hopeless, but there's a way out!
Disable Compilation Directly
If you have SSH access you can log in to your server and run the command:
$ php shell/compiler.php disable
If that doesn't do the trick there's one other option.
Log in to your FTP server and find this file. When you open it up to edit you should see two lines that each contain "define" function calls. Comment those lines out and you should be able to get back in!