The floor vent defaults to the open position and uses vacuum to close when you have "Vent" or "A/C" selected. The vacuum comes in from the engine compartment, goes through the back of the A/C switch panel (there's a vacuum switch that changes position with the sliding levers), then distributes underneath the dashboard. I fought issues with 2 bad vacuum switches on mine (yes, one out of the box was bad brand new).
A chassis service manual will help you diagnose this. You can use a handheld vacuum pump to actuate portions of the system, or you can pull apart the connector behind the glove box door and manually jumper vacuum signal around to various components. The vacuum signal goes in through there, heads to the control head, then distributes back out through a connector there. If you know what the color codes are for, it's easy to jumper.
My bet is you either lost the vacuum signal (hose came loose from the accumulator ball or at another connection), or the switch on the back of the AC control head is bad. The worst case option is that the actuator for the floor vent took a dump, that can't be fun to change. Once I knew I had a vacuum source under the dash, I used a vacuum pump on that actuator under the dash, and when it moved, I knew I obviously had a controller issue somewhere.