Why does my furnace blows out cold air, when it should be blowing hot air?

Check the thermostat to see if the blower switch is set to on. It should be set to automatic. Does the furnace heat at all ? Try that first and post back.

Thermostat is set to heat and auto

The furnace is on heat and auto but isblowing cold air

It is on heat and auto

Brand of unit?

Exact model number?

Yes but still blowing cold air

It was on run not auto, why does that matter it was on run and it was working before