Single Player Mission Glitch, Unplayable, SPOILER ALERT description of mission

On the mission where you are escaping from prison, after retrieving Irish and going up the elevator, the entire level fails to load, with all enemies and npcs constantly dropping off the map and reappearing in a blank white space.  any attempt by the player to move forward into what should be solid prison ground results in falling off a cliff.  anyone else experienced this or have any ideas how to fix it, a reload did nothing.  I can upload a screenshot if it helps