Heater blows out cold air this started 2 days ago.Is it the thermostat. on 1999 Ford Windstar

This problem just started a couple days ago.

Asked by for the 1999 Ford Windstar
A leak any where in the coolant system could cause an air pocket in the heatern core.
2 more answers
If it don't run hot,it's not the thermostat.Check the water level in the radiator and if that don't work then the heater core is stopped up.
The fan is broken