I don’t do a lot of ExpressionEngine work any more, but I still get the occasional email about it. More often than not, a serious bug triggers the uniquely ExpressionEngine response of a completely blank page. Not particularly helpful when you’re trying to debug the issue.
Although there are various settings you can trigger in the admin, I found the easiest thing to do is to add the following lines to the index.php file in your site root, just below the error_reporting(0) line (EE 1.6-7) or the $debug declaration (EE 2):
Reload the affected page to see the PHP error that is being thrown and figure out where to start debugging.
And of course, don’t forget to remove the debug code once you’re back up and running!