They very somewhat brand to brand, but the basics are usually the same. There's a split ring that's expanded by two cones wedging into it top and bottom. When you tighten the expander bolt - not the top cap screw - the ring expands and jams. Your problem is that it doesn't shrink back enough when the pressures off.
This isn't all that rare. See if you can find an "L" or "J" shaped piece of metal you can slide through the ring and hook behind it to give you a purchase to ease it up. I use an old screwdriver that I heated and bent the last 1/4" to a 90 angle. It's my mini pry bar and I use it to pull all sorts of parts up out of small holes.
If your fork has an open bottom you can reach through with a long rod and push the ring up. If not, and you can't find anything and don't want to bend a screwdriver, buy a toggle bolt as I suggested in my prior post. The ring will come up without much resistance, but you have to push it without expanding it.
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