There have been a few posts about this problem on the windsurf general forums. A recent one was titled "a trap for young players" if you do a search for stuck mast you will find heaps of ideas.
Try hot water on the outside section and super cold on the inside near connection. Or just soak in hot water for a fair bit hoping water gets into join and dislove salt and soften dirt.
I'll let you find the one of a 100 different ways to get it apart, but I'll offer you this. Wrap a little electrical tape around the joint from now on before you go sailing, it will never happen again.
