pb blaster, coco cola, tap the stem to vibrate it a bit.

Had you left the stem intact, you could do as Southerland's recommends, clamp the fork crown in a vise, and pull and twist on the handlebars for 15 minutes at a time, let cool, start over.

What kind of stem is it? Wedge or cone?

Once I had to stoop to taking the conical wedge out of a Modolo stem, take a piece of copper pipe and beat it from the bottom of the fork till it broke loose. Use a pipe slightly smaller than the inside diameter of the stem, so you don't expand it with the pounding. I did this because a not to be named bike shop told me that the frame was trashed, and the reason one brings a bike to a shop is to have it properly serviced. Of course the punk kid had never even seen a quill stem and hadn't a clue. He did sufficiently raise my ire enough to enable me to pound it out with reckless abandon.

It worked fine, no damage done to the frame or stem. The copper pipe is useless, though.