Yes, this is an annoying issue but it can be cleared up. What I suspect is happening is the editor is seeing the edited file, not as HTML but as autoformatted text. This is what I think HTML area used in v 1.9.x and for some reason crops up every now and again to annoy me - but will do so even for courses already saved in v2.x. There is this discussion, and the related Moodle Tracker item which may help you understand what is happening. At the bottom of the editing interface there should be a drop-down dialog box which is likely to be reading "Moodle auto-format". If there is, then change that to read "HTML format" then save it. Exit the course, then re-enter, go into the editing screen and it should then be showing the tool bars.
If you are not seeing the drop down dialog, then I suggest you download a new Moodle, unzip it, delete the entire editor folder from your production site and replace it with the newly unzipped editor folder. (This changes the editor folder and erases any possible errors or corruptions.) This technique works, without an issue, due to the connectionless nature of web technologies generally. Make sure you completely delete the editor first though, do not just overwrite.
You may still get the editor but no buttons, or maybe not even the toolbars, but once you change it to HTML format as described about, the problem should clear itself up.
Good luck...
Thanks, Colin! Great advice!
I just migrated 3 sites from a partner service to our own server. Two of the sites migrated with no problems, but the third site had been originally upgraded from 1.9. First the menus wouldn't work, which I had to click to use online YUI libraries to fix it, and then I had no html editor anywhere in the instance. Your advice saved me tons of time. It worked flawlessly. I used a tinymce file from one of the other instances.
Thank you for sharing your knowledge! I am sharing to let other's know this does indeed work.