If you can’t get into your Magento administration area and are stuck with only an error, there is a way to get in by editing a file.

1. Via FTP, browse to /app
2. Open up Mage.php
3. Go to line 116, or do a search for: static private $_isDeveloperMode            = false;
4. Change that so it is true and your basic error page should turn into a detailed error log. Make sure you don’t leave that option set after you’ve fixed your problem.