K you know in the City 17, where you just shot the gravity orb out of that case and it opened that gate for you? Well, I went into the building kiled all the guys, and now I'm lost. What to do?
Bit hard to guess where you are. Have you been following Barney? If so, barney should open some gates for you.
HOWEVER, there is a problaem where Barney just stands still on a pile of rubble. He is stuck. To fix this try walking into Barney from all directions and shooting the ground at his feet and jumping on him. You just need him to move a millimetre then he will continue as normal.
If this does not help then post screen shots. If you are where I think you are, post a screen shot of Barney so I can confirm if this is where he is stuck.
In the first pic, you see those large metal doors? There should be a door next to them going into a small room (with a poison headcrab behind you, watch out for that). You can then use the manipulator to remove the piece of wood on the door acting as a lock, which should open them.