If none of the previous solutions worked 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 might not work. To know which version of Drupal 6 was installed, open the CHANGELOG.txt file. This file is located in your Drupal root folder. The first section of this file indicated your Drupal version.
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.