Originally Posted by JohnDThompson

The stem bolt extends through the stem to a wedge at the bottom. Tightening the bolt presses the wedge against the steer tube and holds the stem in place. Standard practice is to loosen the stem bolt slightly, then strike it from the top with a hammer (a soft face or wooden block on the bolt head prevents damage to the bolt head) to loosen the wedge at the bottom of the stem, after which the stem should come free. From your description, it seems you have disengaged the stem bolt from the wedge, and the wedge is still jammed in place holding the stem. You will want to re-engage the stem bolt with the wedge to be able to knock the wedge loose.

Penetrating oil on both the stem and wedge from below should help, as well as on the bar binder if that seems seized as well.