You should check for the integrity of your ducting system. You didn't say where your furnace is located, but in many installations it is in the attic. In that case, the downstairs and the kitchen floor vents are the most distant from the furnace. If a duct has come loose from the air plenum attached to the furnace, you're losing heat into the attic and none is going downstairs. Your should reattach the duct to the starter collar, and don't just use duct tape but fasten the duct securely with a plastic tie wrap. Then fold the insulation over the end of the duct and wrap with aluminum foil tape, which you can buy in the heating section of Home Depot and elsewhere.
The other thing you should check is to see if there are dampers on the duct line. These are sometimes installed so that the heating technician can balance the air flow. If the nut has come loose, the damper may have accidentally closed, blocking the air flow.