What are the reasons for images going missing without changes made?
There are known issues with the images after the security update of Drupal 7.20. I would suggest you to update the latest release if it is the case.
I would also check if there is a .htaccess file preventing access to the files or folders. I remember that there was a similar issue and it was related with a .htaccess file restricting access to the files under the path where the images are saved.