I had a friends car do this and it had snapped the flywheel bolts...well actually they kinda fell out because he didn't do them up
Move the slave fork yourself and see if it has really little pressure on it and if you can move it freely back and forth by hand obviously somethings up. I would say you gearbox input shaft broke but your saying the starter won't spin engine? Try spinning the motor (uhh flywheel?) with the plugs out and seeing if the pistons move, if they don't you will need to drop the box and see whats up.