If you didn't over-tighten the stem when you installed it, and the bar is still on, you can try twisting it a bit. Be careful not to get carried away if you're holding the fork by the front wheels between your legs, because you can twist the fork blades. If you want to try twisting harder, hold the folk crown in a vise between two wood blocks. Now tou can twist the bar while pulling up. Once you've moved it a millimeter or two, you can drive the wedge down and free the stem.

Back in the bronze age, we made it a point to only tighten stems race tight. That's tight enough to hold for steering, but loose enough to twist in a crash. That protected front wheels and forks from added damage if the wheel swung around until the bar hit. If yours is only race tight, you'll be able to wist it, and work it up enough to free the wedge without too much effort.