That's a quill stem if it still the stock piece. Chances are it wasn't greased properly and the two parts rusted together (both items being made of Hi-ten steel it's no wonder). Thread the long bolt back in untill it's flush with the top then take a rubber mallet and strike the top where the bolt is. If this fails to loosen it you'll need to attack the problem by chemical means. PB Blaster Liquid Wrench etc.