Halo Headquarters

Halo: Combat Evolved question?

Im trying to get out of the Pillar of Autumn, but I can not. Cortana tells me to follow the navigation points and I each Once I finish a door that is open and I have no grenades and shot at him with everything and still can not open anyone can tell me how to open the door?

I thought I had no problems such as Halo, but I just remember I did once, in only his. Is this some kind of door that leads small cage in some tunnels and there were also some of these little guys around her (I do not know what it is called) if the If it would not open the door for me once. The best thing I can advise is to restart the checkpoint or restart the level and we hope that this does recurrence.