Leak in the vacuum hose would be my first suggestion.
Jim

There is a blend door under the dash - likely either the servo that moves the door is out, the wires are corroded, or the cable is stuck. This is a labor intensive problem that cannot be solved without disassembling most of the dash - it is possible that you could physically move the door to allow air flow through the other ducting, but it would be very easy to break something crucial as most of the pivoting parts are plastic. Since winter is coming, I know from experience it is better to have defrost than floor heat. the simplest ( and cheapest) solution would be to buy a small ceramic 12v heater to plug into your cigarette lighter or other 12v outlet until spring, when it warms up and you won't need the defrost - this will also give you the chance to save up some money for the repair, since most shops charge $85.00 an hour or more for their time - and this could chew up a couple of hundred dollars or more!
Hope this helps and thanks for using Fixya!