My kitchen ceiling collapsed in my old flat - just exactly as you describe. Chunks of plaster an inch thick and the size of my (not exactly small) hands. It demolished my microwave - the top was dented so badly that the door wouldn't shut and the waveguide was knocked through the top of the oven bit itself. I'd just done a load of dishes, which were on the draining board - all broken, and the stainless steel draining board had dents in it. It probably (from my neighbour's description) happened ten minutes after I left, just after I'd been washing the dishes...