After either a fresh install of 3.5.1 or upgrading of an existing installation, the editor seemed to disappear so I was not able to add or edit any pages/posts.
After following these steps:
I deactivated plugins, still no help.
I tried different browsers, same problem.
I edited wp-config and added define('SCRIPT_DEBUG', true);
YAY this worked.
I followed the next step which was to disable this.
OH NO - it broke again.
So I added it again and have left it there for the time being but worried I should not have this there all the time?