If the furnace is constantly blowing cold air, it's likely one of two problems. The first is easy to check and fix, the second will likely require a service technician. Start by checking the thermostat, to make sure the
Fan setting is not in the
ON position. If the fan is set in the
ON position, the fan will blow air continuously. Make sure the fan is set to
AUTO, which will allow the furnace to control the blower.
The other reason a furnace blower will run continuously, it that the high limit switch is dead or dying. If the high limit switch fails, it will "fail open". This causes the furnace control board to think the furnace has overheated, so in response the control board turns the fan on to clear the heat. The most common cause of high limit failure is constant overheating, which is usually brought about by dirty filters. You'll likely have to contact an HVAC Technician to replace the high limit switch, since it might be difficult for you to locate the part yourself. In the future, make sure to check your filter and change them as often as needed.