There is a bug on Xbox One where matchmaking will just stop working. There have been a few reports of this, but nobody with the root cause. I have a pretty good idea of what it is and a workaround until it gets fixed.
So, it appears that when the Xbox goes to sleep with Battleborn running, when the player is logged out, the game sometimes loses its ability to talk to the matchmaking server. When you go back into Battleborn, you’ll get some message about how you were logged out, but you can just press A and it looks like everything is fine, but it’s not. From this point onward, you will not be able to find anybody else in matchmaking. You can enter matchmaking queues without getting any errors, but you will never find a single person no matter how long you wait.
If this happens to you, you can fix it by going to the Xbox home screen, pressing the menu/start button on Battleborn, and choosing Quit in the menu. Once you reopen Battleborn, you’ll be able to find games like normal.