Ken
Are there rounds stuck in this gun? if this is the case, take it to a smith!
if not Often the extractor rod has loosened and is binding the cylinder latch (thumb-piece) from doing it's job. Try holding the extractor rod in place (I use a a aluminum letter opener with a rap of electricians tape) partially cock the gun until the cylinder locking lug drops, then spin the cylinder 1 to 3 turns, release the extractor rod and try opening the cylinder. If a loose extractor rod is the problem, a little nail polish on those fine threads should keep this from happening.
Good Luck & Be Safe