Don't open the door

>Try sliding the piece of paper that you ripped off the door underneath the door halfway and then pull it back out. Also, if nothing happens, try knocking on the door.

You go closer to the door and pull the piece of paper you took earlier from your pocket. As you crouch to try and slide it underneath the door, you realise you can't do it, because there is no space between the floor and the door.

You're a bit disappointed, but there is nothing you can do about it. You put the paper back in your pocket and stand up. Then you knock on the door.
You hear the sound of your knocks echoing behind the door for almost a minute, before fading out.
Suddenly, you hear a very loud bang behind the door that makes the whole room shake, and you fall down on your ass.

What the fuck was that ?