We recently had an issue whereby the TinyMCE HTML editor was appearing blank. Here are a couple of things that may solve a similar problem for others:-
1. Reinstall WordPress via WordPress Dashboard ‘Updates’ – click on ‘Re-install Now’ button
2. Search for a rogue .htaccess file in wp-includes folder and delete it
3. Add the following to your wp-config.php file:-
/** Fix for no TinyMCE */ define('CONCATENATE_SCRIPTS', false );
In our case option 3 solved the problem.
Let us know how you solved the same issue if any other than the above!