Turn off the power source or fuel supply to your furnace if it’s still operable. If you can’t turn it off yourself, call a professional immediately. Turning off the fuel supply will prevent any additional damage to the unit, and turning off any electric current will prevent electrical shock.
If possible, move anyone in your home with sensitive health conditions to a different part of the house that is warmer until help arrives. If there is no alternative space for them, be sure to bundle them up in blankets.
Call a technician to assist you as soon as possible.