Hey,
That's a tough one. I can't imagine why it would freeze up like that. Not having any oil should not cause any problems for just the one shift. Nor should having the top end apart cause any problem with the tranny. It sounds like it kinda wedged itself up. Did it feel like it went into gear or did it freeze up half way? Is your crank/piston free to be moved? If so try turning your rear wheel, easily, back and forth and see if you can free it up.
I'm no tranny expert, so I can't tell you much else. I don't think you could have done much damage. Unless the damage was already done on your last ride, and it just no broke. Good luck.