I assume that you're trying to get through the last door in the AT-ST hangar, because I got stuck there for a bit.
What you need to do is go back out of the hanger and get in one of the gun placements. Be sure to kill any remaining AT-STs below. Then, blow up the ion cannons with the gun emplacement. In the AT-ST hangar, or somewhere along there, you should have disabled the forcefields protecting the ion cannons. After destroying the cannons, the door should be opened.