It's been quite a while since I played so I don't remember all the details, but toward the end of the game there's some kind of emergency mission, maybe the base gets invaded. There is a small airlock-type room in the base you have to pass through to get to the start of the mission, when I went through it this time the "exit" door wouldn't open, and even though the "entrance" looked open I couldn't go through it. I tried reloading, loading older saves, etc. but nothing I did would get the game back on track. My only option would be to start over and I didn't really want to do that. So I moved on.