This question already has an answer here:
I ran into this bug myself earlier today, the basic nature of which is that typing in certain tag markup causes the markdown preview to freeze.
I was testing this markup on Anime.SE to see what sorts of things cause errors, and at some point, I did something involving that markup - I'm not sure what, exactly - that caused the editing toolbar to vanish altogether:
Now, I have no idea how to make it reappear.
I initially caused this problem using Chrome 34.0.1847.116 on Windows 7. I tried clearing my cache (for all sites) and local storage (for https://anime.stackexchange.com/), but am still seeing this problem. That said, I don't think it's a client-side issue, because - get this - I'm also seeing the lack-of-toolbar using IE 11! This only happens when I'm logged in, though; when I'm logged-out, the editing toolbar is present and looks normal.
I also caused the same problem on meta.Anime.SE (screenshot).