Have you been monitoring your engine temps during normal heat verses when the heat is cold? If your engine temp is also lower than normal during this no heat issue then it is very likely your thermostat has failed. A failing or failed thermostat will stick open. I believe this was an intentional fail safe design so that if they do fail at least the motor doesn't wind up overheating and the driver will become aware there is a problem (no heat). If the temp sensor is showing normal readings then it's probably something else like a plugged core or air in the system.