Probably the ejector rod backed out as already stated (L/H thread) if the gun functioned OK in the past.
A couple of other small things to check:
With the cylinder swung open, depress the small spring loaded locking pin (the one that is hanging up causing the problem) and make sure it can be depressed all the way flush into the hole in the barrel lug. Once in a great while the spring kinks or dirt/dried oil gets in there and it limits it's rearward travel.
Also look at the same pin itself carefully. The left side of it where the ejector rod closed upon it is obviously chamfered/tapered to move it back during the closing motion,,then it snaps forward to lock. The overall body taper of the pin assists in keeping things tight.
However,,the right side also has (should have!)a very small, very slight chamfer to the forward edge of the pin to assist in getting it passed the ejector rod when the center pin is not quite pushing to the end of the ejector rod.
It's only function is to cam the locking pin back out of the way that last little bit that the center pin either refuses to do or in the case of extremely fast operation will make the function go smoother.