In a couple of occasions, the WYSIWYG editor at the editing window of our Drupal installation would disappear (we are currently in PressFlow, installed a couple of months ago). It's there for those with admin roles, though, so we suspected it might be a permissions thing.
Here are the fixes that worked in our case:
- the first time it happened, we ticked "administer filters" for the roles where the WYSIWYG window disappeared (which was for everyone except admin)
- in the second instance, I ticked "bypass filter restrictions" and the editor came back on. This was when we migrated to PressFlow and had WYSIWYG problems.
Others suggest removing the buttons of apps that you have recently installed (from site configuration - wysiwyg profiles, full html input format) and see if the editor returns. I actually removed all the buttons and added them one by one when I was trying to troubleshoot the second instance (this did not work for me though, so it meant I did not have apps conflicts).
You might also want to check out the TinyMCE FAQ for other suggestions. Hope this helps.