I am assuming that the stem bolt will not drop down and the stem will not move. First try to get penetrating oil to that area. Remove the stem bolt entirely and squirt the oil into the hole. Ideally get a hose of some sort to direct the oil to the outer part of the stem. Even better is from below depending on fork design. It can't hurt to loosen the headset locknut and squirt some WD-40 down the outside of the stem to see if it can penetrate to where the outside of the stem expander meets the handlebar. Then let it sit!

Before you try again remove the wheel from your bike and provide a solid surface on which the fork can rest. A short 2x4 supported by something very solid would work for example. One could also clamp the fork column in a strong machinist's vise (yes, while still mounted on the bike) with wood or similar as solid padding. Put the stem bolt back in and use a heavy hammer and a piece of hardwood, or MUCH better - a 24 oz dead blow plastic faced hammer, and hit the bolt again. If the wedge moves but the stem is still stuck you will need that machinist's vise, as I would then recommend carefully but firmly twisting back and forth to loosen the stem. DO NOT twist the stem while holding the wheel in place with your legs.

If the stem bolt still does move and you can determine that you have a conical wedge rather than a slanted style, take a strong steel rod that can fit in the stem bolt hole and hit the wedge hard around the perimeter to break it free.

Last resort is to remove the stem bolt and saw through the stem near the top of the headset so you can have better access to see and address the problem. If all that fails you have my sympathies. The bike is probably not worth a new fork, headset and stem