Very sad, OK one minute and gone the next. Maybe the jack and stands tipped over.
I grew up in an era where we we used an 'A' frame/block and tackle to remove engines but gearboxes were lowered on to your chest and you did a body roll to lower it to the ground. Refitting involved arching your back, with the gearbox on your stomach, to locate the bell housing dowels. This was all done with two trolley jacks and no support and I went through life working under cars in this manner.
About five years ago I crawled under the truck, had a really uneasy feeling about it and crawled back out again. I went out and bought a pair of axle stands and use them all the time now.
As I've posted on here before, my old car ramps let go on me through rusty joins so be aware of old car ramps as well.