What security patch have you applied? Open console in web browser to see if there is any javascript error which can tell us more about the issue.
In meantime, try following fixes:
1. Edit app/design/adminhtml/default/default/template/media/uploader.phtml
2. Find line:
var maxUploadFileSizeInBytes = <?php echo $this->getDataMaxSizeInBytes() ?>;
3. Add single quotes, so it looks like this:
var maxUploadFileSizeInBytes = '<?php echo $this->getDataMaxSizeInBytes() ?>';
If still doesn't work, try to modify variables maxUploadFileSizeInBytes and maxUploadFileSize:
var maxUploadFileSizeInBytes = '128M';
var maxUploadFileSize = '128M';
If that works, it means that there is some problem with php.ini or you're using HHVM.
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If this response solved your problem, click accept as solution to help others solve this issue