The shower door on my 28A has a small rotating plastic arm about half way up the door frame. You twist the arm so it grabs the grab rail on the first section when the door is closed. The arm is mounted via one screw. I would guess the flimsy arm broke and there is a screw hole visible on your door frame.
I also went to the hardware store and found a handle from a sliding patio door and mounted it to the shower frame just above eye level. That makes getting out of the shower much safer for me. Plus, I hang my bath towel on it.