Seanwalsh

PLZ SOME ONE HELP i cant get passed the mission were you escape the helicopter and you go into a railway tunnel when i get the mission "go deeper into the mine" i do so but when i reach the bottom and go forward i get knock out and then die i cant get passed it and it sucks. and i know you are should not die because i look up vids and when the get knock out they wake up and talk to this guy saying "what are you doing here?" so plz help me.