I last posted on my employer's blog 1/21. Everything was fine at that time. Then I logged in to publish a new post 1/27 and found the visual editor toolbar missing and I am unable to enter anything into the visual text box. I went to the WordPress support and searched for fixes.
I first hoped that something simple like logging in using different browsers (Safari & Firefox) & clearing the caches of each browser might fix the problem. I went through the settings to make sure the Disable Visual Editor wasn't checked - it was not. I have not changed themes, I disabled all plugins, restarted WP, but no change. I went through every minor step I could find on the support site with no luck, so I backed up my files and did the update to WP 3.5.1. Again, it did not fix the problem.
My last attempt at a fix was to follow the suggestion of getextra on a post that was started two years ago. The suggestion was to delete the wp-includes folder and then copying a wp-includes folder from a working version of WP. While I wasn't sure what that entailed, I logged into the control panel, went to file manager and found the wp-includes folder. I then located the same file in my backaups that I do regularly of my site (this was backed up from a time when I had no issues).
Well, even that didn't help. I am not proficient at html, I have been managing the blog for a couple of years, but I've never had to deal with this kind of issue. I've been learning as I go and have learned how to do backups from our WP site as well as from the host's control panel. I don't know how to perform some of the more technical suggestions I saw, though, so it may be necessary to go to the host for support and hope they can help. However, if anyone here has a suggestion I would welcome a different suggestion. Thanks!