During Act IV, Titus tasks the Hero with finding weapons in Cell Block B for the inmates. The Player must go through the Basement and the Laundry Room in order to reach the Control Room. But the door is stuck, and so the Hero must find an alternate route. Just then, the Voice tells them not to bring the weapons to Titus and says that he's not trustworthy. The Hero heads through the prison sewers and through the showers (which are full of slain bodies) to reach the elevator to the Main Control Room.
When you reach the control room and take the elevator you will be no longer able to go back to any previous areas. In addition to this, if you die after you have reached the control room the door you entered from will be locked and you will only be able to proceed through the elevator. If you are trying to obtain all the collectibles make sure you got all the ones in the previous prison areas before starting this quest and make sure you don't skip any before reaching the control room.