The windshield defrost vent is the "default" position for the system. It is found in this position if you have a vacuum leak somewhere in the system. Look for something that is disconnected under the dash or under the hood.
It could also be that your heater vent motor became disconnected from the vent door. This happened to me after I worked on my heater core once. It is accessed from the driver side immediately under and behind the radio.
A forum community dedicated to Jeep Wrangler owners and enthusiasts. Come join the discussion about reviews, performance, trail riding, gear, suspension, tires, classifieds, troubleshooting, maintenance, for all JL, JT, JK, TJ, YJ, and CJ models!