After updating my WordPress to 3.9 I noticed I could no longer type inside the big content box. I also noticed the editor menu above the box was gone. Using Google Chrome / Inspect Element I saw a red error msg that said "Uncaught TypeError" tinymce was trying to load but couldn't. I looked at the right of the element inspector and saw that the div caught had a style of "hide-if-js". Began researching and came across this post.
I discovered that WordPress 3.9 no longer supports TinyMCE but instead uses WP-edit. I never installed TinyMCE but it made me realize that another plugin may possible be built on top of it. I one by one deactivated the plugins I had installed, checking back after each one to see if that fixed the problem. After the 3rd plugin I deactivated, the editor menu appeared again and I could type in the content box! (I did go back and reactivate the 1st 2 plugins since I knew these were not the problems.)
I hope that helps. By the way - the plugin that was causing the problem was Foter but I am sure there are several other plugins out there causing the same problem.