We have a gas furnace with two thermostats and forced air on the second floor. The heat works fin everywhere but the second floor. The temperature is dropping and the registers have cold air blowing out of them. Any suggestions on what the problem could be?

If you have 2 thermostats, you have a zone valve and controller somewhere. If you can find the zone valve to the second floor, make sure it is opening when heat is called for. If it doesn't work, you have a bad zone valve, bad controller board, bad t'stat or possibly a disconnected riser pipe or a bent zone valve not allowing it to open. Are you sure the second floor t'stat is set for heat?