I had just come up from my mineshaft, having found 2 diamonds. enough for a sword. I made one, and then went back to my mine carts. I got in one, hit the button to go (It was a simple machine, I've used it hundreds of times perfectly.) and expected to be flung down the miles of track I put to the end.

Something went very, VERY wrong. actually, 3 things.
1. My minecart dissapeared and my player still "sat" in it, though I could move.
2. THE WORLD DIED. everything de-spawned. all the chunks, animals, mobs, everything.
3.I of, course, fell. but I landed ON the void. my map (one of the few mods I had on) said that I was on layer 0.

I looked at some things, including hacking to put me into creative so I could fly, but nothing got me back to normal. It only teleported my far away when I re-loaded the game

I don't know what happened, so I cannot tell you how to re-produce it. my mineshaft worked fine when I got home. attached are some screenshots I took. If you'll notice, I have a pair of boots and a saddle when I re-load the world. I did not have these before.