Too many people in this thread don't understand the difference between an expander plug(used in a carbon steerer tube) and a star nut(used in a metal steerer tube) and keep giving advice as if the OP has a star nut stuck in the steerer. There is no star nut involved here. The expander fell down into the bottom of the steerer tube and the tube diameter probably gets a bit smaller at the bottom, so its stuck. Turning it over and hitting it may free it or he may need to hook it with something to pull it out the top.