"500 Internal Server Error" error

If you get an "internal server error" message, try the suggestions at the WSOD (White Screen of Death) page. Some of those problems will produce an "internal server error" message rather than a blank screen. (This may depend on your configuration).
http://drupal.org/node/158043

This is true especially if you get this message after attempting to enable a module, and find that the site seems to work okay, but the admin modules page no longer works.

If none of the above work you could try the following. But this should be try as a last resort.

Steps:

1. Backup your database and files so you can revert if the following is not working. Read more about backups at http://drupal.org/node/22281
2. Delete all Drupal core files. But do not delete other appropriate files though. For example do NOT delete the settings.php file located under sites/default/settings.php. And do NOT delete the all folder and its sub-folders located under /sites/all
3. Download a fresh drupal-X.XX.tar.gz at http://drupal.org/node/3060/release Make sure you download the exact same version that was installed before the error. For example if you were using Drupal version 6.12 then you should download Drupal version 6.12 tarbal. Not a more recent version. Otherwise it will not work.
4. Install the fresh Drupal core. To do so read the README.txt file located inside the Drupal tarbal.
5. Go to http://YOUR-DOMAIN-NAME-HERE.com/update.php
6. Follow instructions on screen. Start by clicking on Continue button.
7. If not working revert to your backup files. The files from above step 1.