I have experienced this error several times, both on Firefox 3.6.x (right now 3.6.3) and IE8:
After creating a post and working on it for a while the Visual tab will show the HTML code. Changing browsers will not help, that particular post will still be visualized in HTML under Visual.
And the menu bar (the one under Upload/Insert ) also disappears.
Right now I have to stop working on my post, a few days ago I had to let it go and when I went back a few days later it was ok again. I cannot, though, take a 2 day break every time I need to post something new.