If you can, turn the knob, use your pointy object of choice to press the button, and slide the knob off. If you can’t get the knob off and you have access to the hinges on the door, your screwdriver ...
The knob on the non-locking side of the door should have a small hole in the middle of it. Inside this, there’s a small reset button you’ll need to push. This will unlock the door, but you’ll need ...