please,I'm stuck(4th level);thanks

[guerro]

I hope someone will be able to help me.
In the 4th level ,just after the dream sequence of the flooding water in a room with a guy working on an pumplike engine ,I climb down a stair that collapses .After fighting a zombie guy I 'm in a frozen room .I thinkIi am supposed to reach a door upstairs where is a vent but how? There is also a hatch that doesn't open near the pumplike engine.
thanks