The stem is seized in place and the stem bolt has been removed... this is not safe to ride as the stem could release itself at a very inopportune moment.
You need to find a stem bolt that will thread into the pinch nut that is probably jammed in the bottom of the stem and give that a tap with a hammer to see if that releases the stem... a lot of people don't know how a quill stem releases and will remove the stem bolt thinking this is all that is required.
Or they will unscrew the top bearing nuts...
If tapping the pinch bolt does not yield a favorable result, you may have to treat the stem with some penetrating fluid as well and see if that causes it to release.