Most likely the extractor rod has come loose.
The extractor rod can come loose and if it unscrews enough it can cause the cylinder to lock up. You can check this by seeing if you can turn the extractor rod manually (it may be a left-hand thread).
If that doesn't work, you'll have to remove the sideplate to see what's tied up the gun inside. Using a proper sized screwdriver blade, carefully remove the grips and the sideplate screws. Note which screw goes where, especially the lower-front screw as this fitted screw retains the cylinder yoke. Once the screws are off, use a screwdriver handle to rap the grip frame to loosen the sideplate (do NOT pry it off).
