Bianchigirll is right. Your Nishiki uses the odd 21.1mm stem diameter. I've seen several bikes with these stems and all were relatively short. I suspect you are at the max height right now. There are many threads on these stems and what to do about them. As one who has been through this with a 1981 Schwinn Voyaguer I can tell you that your options are:

1) Live with your current stem
2) Scour eBay for a 21.1 (typcially marked .833) stem that is long. I've seen them on eBay, but rarely.
3) Buy a new 21.1 extra long stem. They are on eBay and online retailers. They are low quality, heavy, steel and hideous looking but they work. Attached is a photo of my bike with this option.
4) Find the stem you would use in a perfect world (I used a Nitto Technomic). Next, find a small machine shop and ask the proprietor very nicely to put your new stem on the lathe and turn it down to 21.1mm. Be sure that you and the machinist are in agreement as to how high up the stem he will reduce the diameter. There are some people who will tell you that reducing the stem's diameter this way is unsafe. These people know nothing of what they speak. Do not listen to them. If you need evidence look no further than your original stem. It is a 22.2mm diameter stem that has had a portion reduced to 21.1 on a machine. I asked Yellow Jersey to do this for me and they refused on safety grounds. I don't blame them for not wanting to be liable, but that doesn't change the fact that their concerns are unfounded. See photo of my bike with this option.

As for removing your stuck stem, try the following. Remove the front wheel. Hold a 1"x1"x18" piece of wood or similar length of 2x4, or axe handle (or. . you get the point) between the fork blades at the crown. Use this as leverage and twist the stem by grabbing it around the handlbar/clamp. Be careful and use caution. This should release the stem. If not, then it's REALLY stuck and you'll need to do a search for the myriad threads on this topic.