Sometimes buttons from WordPress visual editor may disappear or start showing blank white spaces instead of buttons.
Clear the Browser Cache
Upon our search, we found users suggesting that clearing the browser cache worked for them. The first thing you should try to do is clear the browser cache or use another browser. This could very well be an issue with your computer.
If this does not fix your issue, maybe the issue is with your install?
Replacing TinyMCE Scripts
Maybe this issue happened after you updated? Well, its not likely, but maybe something went wrong in the update process. Try replacing /wp-includes/js/tinymce/ folder with a fresh copy.
Fix with wp-config
If none of the above tricks worked for you, then open your wp-config.php file and add the following line at the very top after the php opening tag.