"Page Not Found" error on ALL pages of Drupal 6 website

If you're experiencing the "Page Not Found" error on every single page of your Drupal 6 site, it is possible that your menu router data got corrupted or deleted. This is most likely to happen when there is an error that occurs during the process of enabling a module. See this bug report: http://drupal.org/node/238760 The problem should be less likely to occur after the Drupal 6.3 update is out.

To fix this, your first step should be to try to figure out what might have caused it and to undo those last few modifications that you made to the site. Typically, this would involve reverting to old versions of modules, or even disabling all non-core modules. Since the site is not accessible, you may need to disable the modules by SQL. Set the status field of the suspect contributed module(s) to 0 in the {system} table. There are plenty of threads in the forum that discuss how to do this, if you are not familiar.

Once you've done that, you just need to rebuild your menu system to give you access again. The easiest way to do this is to run update.php. Even if there are no updates, running through the process will lead to a the menu router being rebuilt.

If for some reason running update.php doesn't work, you may need to temporarily alter some code as outlined below. This should be a last resort, and is not normally needed.

To rebuild your menu, find the menu_execute_active_handler() function inside /includes/menu.inc (approx at line 334) and force it to call menu_rebuild().

I.e. temporarily change this:

to this:

Now if you try to access your site again, it will rebuild the menus and you should be back to normal.

IMPORTANT: Don't forget to undo your changes to menu.inc though, otherwise your site will suffer a huge performance hit!