Been a long time since i've done that mission, but it sounds like the mission area you are in is bugged (Occasionaly you'll find doors you can't get into). Best thing to try is starting the mission in a different area, so hopefully the mission area will be in a different type of building. Alternatively sometimes, leaving the mission area, hardlining somewhere else, then coming back fixes the problem, but it's not fool proof.
Anywho, hope you get it sorted soon