1. Deactivate all plugins. Yes... ALL plugins.
2. Switch to the default 2014 theme.
3. Manually empty and refresh your browser cache.
Only after you perform all three steps.. will you see the content editor load properly.
1. You have to deactivate all plugins to rule out possible interference from a plugin.
2. You have to switch to the default theme because... well... we know it works.
3. You have to manually empty your browser cache because the tinymce editor is notorious for holding onto and serving cached files from the server.
Now, after your editor is working properly again... reactivate your theme.
If it continues working... it's not your theme. Begin reactivating plugins (one at a time) going back to check the editor.
You will eventually find the faulty plugin (or theme) that is preventing the editor from displaying properly.