A good old movie trick explained in pictures.
Choose your largest, most easily flexible credit card.
Slide the card into the vertical crack between the door and the frame (where the lock is). Insert the card as far as it will go at a perpendicular angle with the door.
Tilt the card so the side that's closest to you is almost or is touching the doorknob. Push it until you feel it slide in more.
Bend the card the opposite way, forcing the lock to go back. Quickly open the door. It is a good idea to lean against the door while doing this, which will help it to pop open.