How to open this locked door
That type of latch rotates (there is a half-cam inside)
That square hole you see is engaged by the shaft on the missing knob, and rotates that piece.
You can use anything to turn that either way. A screwdriver with a bit too big to fit straight, but will fit in 'diagonally' will do the trick...
a quarter turn should open it fully.
This is provided it is the same as the kind I am trying to describe, from what I can see in the picture.
Also, after you get in, when you try to put the pieces back together, make sure the key works BEFORE you close-up and lock yourself out.  It is often possible to get the inside knob and outside knob together wrong by 90º, and then you will probably not be able to rotate the key one direction, and/or be able to turn the lock-tab inside both ways.  Check the knobs and keys and lock tab function, with the door open, for proper operation BEFORE you close the door...