I have had this problem all day: in the edit posts / pages screens the visual editor was not showing up I googled the problem and got a variety of solutions which didn't work for me.
I found a solution which fixed the problem right away and thought I would share it on here to help anyone else out with the same problem as me. I cant guarantee it will work for everyone but I hope it helps some of you out....
In Google Chrome I pressed f12 to bring up the developer tools window and clicked the icon to bring up the error console and found that there was a missing file: 'en.js' from the wp-includes/js/tinymce/langs directory.
In the directory there was a file named: 'wp-langs-en.js'
I renamed this (obviously making a copy of it first, just in case) to: 'en.js' and uploaded that to the wp-includes/js/tinymce/langs directory on my website and hey-presto everything was back to normal :-)
Like I say it may not help everyone but if you haven't tried it out then possibly worth giving a go to see if it works!