Yesterday, I encountered the weird problem. Cron on one of my Drupal based web sites got stuck. I was checking my search index and realized that new content isn't being added to Drupal search database. Cron isn't launching and Database logger says that: "Attempting to re-run cron while it is already running". This is a common problem. So what should you do to fix this?
Try to make these simple steps (previously, backup your database):
Hope this will help you to fix the problem and save up some time. Good luck!