Maybe the front lens standard rail has extended upward beyond the edge of the opening of the dropping front.
Try to rotate the knurled ring with the flat of your finger. Since I don't know if the geared shaft (pinion?) of the lens standard focus rail is over or under the rack, you should try it both directions and see if the front then falls open.
My guess is that you would have to place your finger on the bottom of the knurled ring and try to roll it upward...