The most common cause of that symptom in recents releases is due to a security enhancement in an updated version of the wysiwyg editor that is bundled with CiviCRM, that disallows symlinks.

If that is the issue, you can generally fix it by updating the corresponding path in http://example.org/civicrm/admin/setting/path?reset=1 to its non-symlinked version.

If that's not the issue, then reviewing the path and url settings might shed some light on the problem. And if none of that gives you a quick solution, I'd recommend looking at your server logs which should be instructive (either the civicrm log and/or your web service [apache/etc] log)

You might also want to review similar posts here:

 - https://civicrm.stackexchange.com/questions/2769/why-cant-i-upload-images-to-mailings
 - https://civicrm.stackexchange.com/questions/4733/error-uploading-an-image-in-email-wysiwyg
 - https://civicrm.stackexchange.com/questions/13043/inserting-images-in-mailing