Very recently happened to me on my 686. Can you get the cylinder out?
If not, move around the cylinder pretty hard until it shifts. It gets stuck in a state where it never quite locked in, but it prevents the hammer from coming back as it can't turn over the cylinder.
And if that still doesn’t do it, you can put a very small flat-head screwdriver into the end of the cylinder rod and manually push that locking piece away (towards the muzzle), and the cylinder will be able to be pushed out.
Hope that makes sense