The following solution fixed the problem for me.

  1. Empty the /var/locks/ directory of your magento installation directory and set the permissions of that directory to 777.
  2. If the above doesn't work, then run the script /shell/indexer.php

This article originated from a work by Janzell Jurilla and is licensed under a Creative Commons Attribution 3.0 Unported License. The original work can be found here.