I have a door in my 1910 house that is stuck shut. This just happened the other day, the doorknob was working fine before that...

The doorknob itself will turn normally, but it isnt engaging the latch/bolt. I can remove the knob and square shaft (b/c the setscrew on the otherside was already missing), and I can physically turn the square "receptacle" (for lack of a better term) with a screwdriver. The receptacle turns freely, but doesnt engage the latch.

I need some suggestions for opening the door?? Any help is appreciated!