Disable your plugins one by one, starting with your cache plugins and see if it fixes the issue.
I was having the same issue. In my case, my cache plugin Autooptimize was breaking both the tinymce visual editor and the quick tags in the text editor.
To fix both, I excluded the following JS scripts in my cache plugins:
js/tinymce, js/backbone.min.js, js/underscore.min.js, js/comment-reply.min.js
If you use Autooptimize then add the above piece of code here
Settings > Autooptimize > JavaScript Options > Exclude scripts from Autoptimize:
