Dreaded blank page of death

A search for "blank page" of the Drupal.org domain yields about 37,000 results.

This is probably one of the more common and frustrating problems for Drupal admins.

I've had the problem dozens of times myself. For no apparent reason, pages will simply come up blank. Drupal returns nothing at all, not even a set of empty HTML tags. As a developer, I'm used to seeing broken sites and fixing them. But the typical site administrator may not have the knowledge or experience necessary to debug something like a blank page.

The first thing you want to do is gather all available debugging info. The two primary places you'll find this information is in Drupal's logs (Administer >> Logs) and in your Apache log files (/var/log/httpd on Linux systems). If you're lucky, these logs will provide obvious clues as to what's wrong. Whether or not your able to gather any clues as to the source of your problem, below is a list of the most common causes of empty server responses:

1. Memory: In your php.ini file there is a value called memory_limit, which is set to 8 megabytes by default. If you've installed a lot of modules, you can easily exceed 8 megs on a given page request. The solution is to increase this limit (I set mine to 32 megs because I have lots of memory on my host and few domains being served on it). If you're in a shared host environment, you won't be able to modify this value. In this case, you'll need to disable modules or get a new host.
2. Buggy Modules: If you're problem began after installing and activating a new module, move the module's directory outside of /ROOT/modules/. Experimenting with modules is probably the easiest way to "break" your site. My suggestion is to test all modules on a non-production Drupal installation. Move them to your production site only after making sure they work properly (This message approved by Captain Obvious).
3. Incompatible Modules: Using 4.6 modules on a 4.7 site is also a great way to break stuff. Make sure you've installed the module versions that match your Drupal version.

4. Broken Themes: A buggy theme can easily render a site unusable. My suggestion is to preview themes at the Drupal Theme Garden or on a non-production Drupal installation. If it's too late for that and you're experiencing blank pages, you should try setting the theme to a "safe", stock theme. If you cannot access your administrative pages (http://.../admin/themes), you will need to move your custom theme from your /themes/ directory to another temporary location. Alternatively, you can run this SQL command on your Drupal database: UPDATE variable SET value = 's:10:"bluemarine";' WHERE name = 'theme_default';

I don't believe I've ever seen the 'blank page' problem in a production release of Drupal core. The problem only crops up when installing and playing around with new modules. So the bottom line is be very careful when experimenting with modules. I almost always use a sandbox installation of Drupal to test modules before installing them on a production site. (updated: Thu Jul 13 14:11:27 2006)