This is a common problem with S&W revolvers, usually caused by the extractor rod loosening up during firing. When the rod gets loose, it backs-out of the cylinder slightly, making it "too long" and preventing it from coming free at the front (under the barrel, at the front lug) when the cylinder release is pushed forward. If it gets REALLY loose, it will prevent cylinder rotation and firing, as well as opening.
To fix it, first you have to get the cylinder open (on an empty gun, of course; double check, please). Once open, you can use padded pliers (thick leather works best) to tighten the rod, with one VERY IMPORTANT NOTE: the rod is a left-hand thread on most guns made after WWII, so you have to tighten it in the OPPOSITE direction of a normal screw-threaded part. If you keep turning it and turning it and it's not stopping, you are probably turning it the wrong way, and if you turn it too far, it will pop out of the the cylinder, and you might loose it (or other parts it holds in place). Get it good and snug, and check it regularly. If you notice the sticky cylinder release problem returning, stop shooting and tighten it before shooting any more (again, on an unloaded gun only). Be careful about over-clamping it when tightening the rod; I've seen a few pinched/crushed by folks using Vise-Grips, or an actual bench vise.
If it becomes a regular problem, some folks recommend removing it, cleaning and drying the threads on the rod and inside the cylinder, then adding a tiny dab of blue LocTite to keep it from working loose in the future. This can interfere with needed disassembly later on, though, and it can also be overdone (put too much screw glue in/on the threads, and you might stick something inside the cylinder that you didn't want stuck).
Hope this was helpful.