Ejector rod has likely backed out.
Remove the cylinder and put a few empty cases in the chambers to support the extractor star. Clamp the end of the ejector rod in a leather padded vise and snug it up by turning the cylinder. Take care to not put any side pressure on the rod as you grip the cylinder and remember that it’s left handed threads. If it loosens back up you may want to remove it and put a small dot of BLUE thread locker on the threads.
Could also be that the center pin has a burr peened onto it making it stick inside the ejector rod. If you can push on the center pin when the cylinder is open and it will stick it may have a burr. If so pull the cylinder out and once again using empty cases for support and a padded vise unscrew the ejector rod and pull the center pin out. The front end will have a small burr mushroomed onto the end. Carefully bevel the end to remove the burr without shortening the pin. This situation would be rare on a 357, more common on heavier recoiling calibers.
