How to fix Magento 1.8 Admin Blank Page

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