Happened to me once with a different pistol.
What I think happened, is that you had a dud round with no gunpowder in the chamber, and when you fired the shot, the primer pushed the bullet far enough to get into the rifling, but not enough to get all the way out of the casing. Now when you pull the slide back it only moves 1/8 of an inch because the extractor is holding the casing, the casing is holding the bullet, and the bullet is stuck in the barrel, so the slide and barrel are being held together by the empty case/bullet.
I got the pistol apart by hitting the front of the slide/barrel with a rubber mallet, but you can also try hitting the bullet back into the casing with a peice of wood through the barrel, if this is the problem.
Though that wouldn't explain why the decocker is not moving.