I expect that what has happened is that the ejector rod has come unscrewed slightly. There is a simple way to re-tighten the ejector rod but it's not completely safe to do with any unfired rounds in the chamber. If you haven't fired it until it's empty, I would suggest you do so before proceeding.
To tighten the cylinder rod you pinch the knurling on the end with a finger nail so that it cannot turn easily. Then you cock the gun and uncock it prepeatedly to dive the cylinder around. When the knurling starts to shave your fingernail the ejector rod should be tight enough to permit you to open the cylinder normally.
Once you have the cylinder open, I would suggest that you remove the yoke and re-tighten the ejector rod a bit tighter than when you installed it while leaving the cylinder loaded with the fired cases in place. BTW, I use an old bit of leather folded over the knurling and pinched by some pliers. Then I use the other hand as a slip clutch by grasping the cylinder firmly and letting it slip once I've put enough torque on the ejector rod. If it shoots loose at the range, I just squeeze a bit harder the next time I tighten it up. After a few cycles of this, you learn how hard to squeeze the cylinder while tightening the ejecotr rod so that it won't shoot loose.
