I am on the mission where you are in the lab and someone named Frank is surrounded by zombies and you have to save him. I started it but I died so I went to do another mission. Now when I try to complete this one the doors to get to him are closed and there is no way to open it. I have completed everyother mission and tried everything but it still doesnt work. What do I do?

I am on the mission where you are in the lab and someone named Frank is surrounded by zombies and you have to save him. 

I started it but I died so I went to do another mission. Now when I try to complete this one the doors to get to him are closed and there is no way to open it. 

I have completed every other mission and tried everything but it still doesnt work. 

What do I do?