If it is a M28 no dash, the extractor rod threads are right hand. The change to left hand came with the dash-1 in 1960. There's not much that can prevent the cylinder from unlocking except for the extractor rod backing out. When you press the latch release forward, can you see the latch pin at the front of the extractor shroud move forward? Normally, it should be just barely past flush, but will move about 3 mm when you press the latch to unlock the cylinder. The forward end of the knurled extractor rod should have a few mm of space between it and the forward edge of the shroud cutout. If it looks pressed to the end of the cutout, it has probably started backing out of the cylinder threads, and the forward latch can't move enough to release.
Try holding the extractor rod so it can't easily turn. (easier said than done, you can wedge something made of wood or plastic, but nothing metal) between the rod and shroud where the knurling is. With your strong hand (shooting hand) grip the gun and pull the hammer back far enough to retract the cylinder lock pin so the cylinder can be rotated. NO NOT COCK THE GUN. With the cylinder free to rotate, turn it clockwise as seen from the rear of the gun, which is opposite of its functional direction. Make sure the extractor rod is not turning with the cylinder. If the cylinder seems difficult to turn, it should become easier almost immediately if the rod is threading back in. Don't force it if it refuses to turn at all.
This should cause the extractor rod to screw far enough back in to allow the cylinder to open. Once open, you can finish screwing the extractor rod back in. There is a tool available that clamps to the knurled end to provide torque to tighten the rod, but you can use a drill chuck, HAND TURN ONLY. Don't use thread locker, and before trying to tighten the rod, place at least three empty cases seated into the cylinder, which will absorb the torque and not damage the slot in the extractor or the pins on the back side of the extractor star. Also be careful not to place side loads on the extractor rod, as it is easily bent.
I have done this procedure a couple of times on one of my revolvers which has some difficulty keeping the rod threaded in after a long range session. Go slow, be patient.