I realize this is a very old thread, but this exact issue happened to me today so I'll share the solution that worked for me.
First off I'll clarify what specifically went wrong for me: I have a 2017 Bolt, and when I went to open the rear driver side door from the outside, instead of the door opening, the triangle piece that's to the right of the door handle came out of the door and fell to the ground. The door handle itself stayed attached on the left side, but came all the way out of the door on the right side, and the door did not open.
I was able to open the door by pushing the right side of the handle back into the door, and then pushing the entire handle toward the front of the car. That re-engaged some mechanism so when I pulled the handle out (as you normally would to open the door) the door opened. But without that triangle piece keeping the handle from moving backward, it was easy to mistakenly pull the handle toward the back of the car, disengage that mechanism, and end up right back where I started.
To get the triangle piece back in and staying in, first follow the first sentence in the paragraph above so the handle is in its proper place. The triangle piece will slide back into the door very easily, but will slide right back out with no resistance which is the problem. With the triangle piece out, if you look into the hole where it's supposed to go, looking toward the back of the car, you'll see a bolt with a nut on it. You need to adjust that bolt. The bolt's head is accessed through the frame of the door, so open the door and look for a round piece of rubber at about the same elevation as the door handle. You can take the rubber piece out with your fingers or a flathead screwdriver. Inside the hole you've uncovered is the head of the bolt. By sheer luck, I had a T-25 star bit screwdriver with a 6" shaft (the bolt head is pretty deep in that hole), which worked perfectly. It cost maybe ten bucks at Ace Hardware, so even if you don't have one you'll save money by just buying one instead of going to the dealer. Slide the triangle piece back into the door, and tighten the bolt a few turns. I stopped before I encountered any significant resistance.
Now when I try to pull the triangle piece out it won't budge at all, and when I pull the door handle the door just opens. Problem solved!