The furnace is blowing only cold air, I tried to turn off the furnace, but it keeps blowing even in the off position on the thermostat

You really should call a tech. It is the cam stat/limit switch.

With the newer furnaces the blower will run all the time if it faults out. You can turn the power off going to the furnace and then back on. That will reset the control board. If it's a dangerous problem the system will not try to relight and will give you a code in the form of a blinking light on the control board.

I don't see where it says that it is a newer furnace. Any way the cam stat will make fan run even when it is set to off.