Excellent post on how to troubleshoot TinyMCE (the WordPress editor) by Andrew Ozz. WordPress is great when it works, but a broken editor is like a pain in the neck (or a plantar fasciitis in the foot). Luckily there are a few things you can try to get your editor back

Here’s what he suggests:

There are a few steps that would help with the troubleshooting if the editor doesn’t start or work properly:

1. Make sure the “Disable the visual editor when writing” checkbox in your profile is not selected.
2. Whitelist or set your blog as “trusted” in your firewall and antivirus program.
3. Disable Gears, clear your browser’s cache, quit it, start it again, go back to the write page and force-reload it several times, while holding down Shift (Firefox) or Ctrl (IE). In Safari select Clear Cache (from the Safari menu on Mac).
4. Try another browser and/or another computer.
5. Disable all plugins, clear the cache, restart the browser and try again.
6. Delete both wp-admin and wp-includes directories and upload fresh copies from the WordPress installation package.
7. And finally install Firefox or Opera, note any Javascript errors, especially the first one and try searching on the support forum for a solution. If no solution exists, open a new thread including the error.

The advice is written specifically against WordPress 2.7 but the advice will apply to most cases regardless of which version of WordPress you’re using. Read it here