If this somehow happened on your flight, it'd be bad news. The plane would depressurize in 0.5 seconds, and everything that's not tied down would be sucked out the door. It sounds pretty much like how you see it constantly in movies. Additionally, the plane might start to break apart, and there's not a whole lot of oxygen that high up in the air.
It's not a great situation. But you shouldn't worry about it. There's so much pressure inside the plane that it would take the Incredible Hulk to open the door. The doors are additionally secured by electronic or mechanical locks.
Moreover, plane accidents are pretty rare. And recent data is even more encouraging. 2016 was the second safest year for flying.
To get all the details on what happens when a door opens, and why you probably shouldn't waste a spare thought on it, watch ASAP Science's video above.