Error 500 in your Drupal? Don't panic.... well, a little maybe, but be sure you'll find a way to solve it.
open your index.php in root Drupal and add:
Reload the Error 500 page and, evoilá:
Fatal error: Call to undefined function invite_page_title() in /[.....]/httpdocs/includes/menu.inc on line
This is saying that there is a problem with a module. In my case, I disabled invite, and seems to cause problems. Before the show php errors trick I was blind, but I disabled several modules and started to have problems, so I had no way to know which one were giving us problems and, so the, how to solve it.
Ok, if this is the scenario (not able to even login in your site to disable modules), then drush is simply magic. Install it and:
drush --uri=website.com enable invite
try now. If the error persists, it is probably that it has changed and now it shows you another module or another kind of problem. That's is, then, another story :-).
Sometimes you'll need to clear your cache or make some other operations, only availabe via admin interface... well, not exactly, drush also can help you, for example, with a "clean drush" For more information visit: http://groups.drupal.org/node/28088