Ok, on to the solution::
So, you left compilation mode on, but cannot reach the backend to switch it off. If you have SSH access, you are in luck.
Step 1: Check current compilation status
$ php -f shell/compiler.php -- state Compiler Status: Enabled Compilation State: Compiled Collected Files Count: 6042 Compiled Scopes Count: 4
- Disable Magento compilation:
$ php -f shell/compiler.php -- disable Compiler include path disabled
- Clear all compiled files
$ php -f shell/compiler.php -- clear Compilation successfully cleared
- Ensure that compiler is disabled now by checking its status:
$ php -f shell/compiler.php -- state Compiler Status: Disabled Compilation State: Not Compiled Collected Files Count: 0 Compiled Scopes Count: 0
Step 2: Clear all cache in Magento
Via FTP or SSH