I had my boxfish get stuck between between rocks to. When realizing my boxfish didn't greet me, I went on a search and was lucky enough to find him before it was to late. WHEW! Then I rearranged all my rock.
My guess is... since boxfish have bony plates instead of scales it makes them less likely to make it outta tight rock work. Not to mention that they are also weak swimmers.
Sorry to hear about your loss. I bet your boxfish was a cool fish.