Sometimes buttons from WordPress visual editor may disappear or start showing blank white spaces instead of buttons.
This problem may occur when concatenated JavaScript is not working. It may also caused by missing or corrupt TinyMCE files, or conflict with some other plugin that modifies or extends the TinyMCE shipped with WordPress.
Solutions: a) Clear the Browser Cache
b) Replacing TinyMCE Scripts
c) Fix with wp-config
<?php
define('CONCATENATE_SCRIPTS', false);
?>