I am only going to write a short post on this but thought I would blog it as it caused be no end of hassle this evening, and hope I can save someone the pain that I went through when the WP visual editor disappears.
First I am sure there are probably more reasons why this would happen but after I looked at all the coding technical solutions the final reason was much more simple.
It is definitely worth checking this solution first before you go down that route, I found the answer deep down in this post on WordPress after trying many things.
All I needed to do was log onto phpMyAdmin and check the wp_user_meta table for the meta_value rich_editing for the user that is affected, if it is set to false then set it to true.
For some reason even though the profile said to use the visual editor, the setting in the DB was still false. Once I set it to true I got my editor back.
Hope this helps, please share or comment if you found it useful!