Don't know what vehicle you have but, sounds like the duct work came apart for the middle vents.
Yes vents can be in the floor instead of the wall.
Cold air return vents are the ones on the wall
the middle vents only supply air from the ac unit using a separate in dash fan. the floor and defrost vents come only from the heater
The a/c - heater controller is out...........
The flap that directs sir to the selected vents is jammed or broken...or the switch that controls it is.
Warm(er) air rises - if vents were high up there would be little circulation without fans.
some thing is probley cloging the vents take it appart and ckeck.
The selector mechanism needs repair or the doors inside the vent are jammed.
the vents work because of the heat which can make the hot air balloon go up
The most likely cause would be a vacuum leak at the rear of the engine or a defective selector switch.