I would think the problem is in the plastic sleeve between the inner and the outer tubes. (the dirt got in there and also the plastics shrinked when exposed to high heat) Put a few drops of oil around the top edge of the sleeve and spray some WD40. Press hard on the handle bar to see if the inner tube moves. If it doesn't, you have no choice but to disassemble the fork. Use a file to enlarge the inside diameter of the sleeve. Hope this would help.