I was going to take the top cap off my thompson stem last nite, it started to unthread and then it stopped coming up! I keep turning, it keeps spinning, but it won't come out! I have not had this issue before and I am not sure what to do?
Look up the steerer tube from the bottom. You'll probably need to shine a small light up the tube in order to see what's happening. Then, have an assistant turn the top cap bolt. Watch closely to see what's happening.
If you're lucky, the entire star fangled nut is turning. If that's happening, you should be able to jam something long and skinny up the steerer tube to help hold it in place. (E.g. get a piece of rebar, sharpen one end and then jam this sharpened end in between the fins of the star fangled nut...)
If you're not so lucky, the portion of the star fangled nut which bites into the steerer tube is still stuck in place, and only the center part is turning. Of these two possibilities, I think the second is more likely. You might try the same trick as above in the hope that applying sufficient force to the tined portion of the star fangled nut will hold the center in place. If that doesn't work, get a steel rod long enough to reach the star fangled nut from the bottom of the steerer tube. (You'll want a few inches to spare at the bottom.) The diameter of the rod should be the same as that of a top cap bolt. Use a die to cut threads in the end of the rod and then thread the rod into the bottom portion of the star fangled nut. (This assumes that the top cap bolt is not protruding through the bottom.) Place some sideways (or downwards) pressure on the rod and then unscrew the top cap bolt. The idea here is to hold the spinning center portion in place so that the top cap bolt can be removed.